[LeeroyHannigan]

Closes #492, closes #495. Based on #491 (the documentation site); retargets to main when that merges.

What changes

The Install section had six pages that each cloned, built, initialized and started a server from source, with the platform packages, the TLS steps and the AWS CLI setup repeated on each, plus a 1,461-line walkthrough carrying the manage command reference, runtime settings and tuning beside the install steps. They are replaced by one page per path and one page of first requests:

• docs/install-from-source.md: prerequisites, database install per platform, build per backend, init per backend, verify, serve, smoke test, upgrade after git pull, tear down, troubleshooting. Two steps the old pages had wrong: MongoDB init reads the connection string from a seed extenddb.toml and needs --overwrite; Cassandra init fails under the image's default AllowAllAuthenticator.
• docs/first-requests.md: tables, items, batches, query and scan, transactions, streams and health from the AWS CLI, behind a table giving the endpoint, certificate and credentials for each of the three ways of getting a server.
• docs/getting-started.md becomes an eleven-line pointer, off the site, so links into the repository still land.
• Removed: manuals/04-quickstart-setup-guide.md, 08-install-linux.md, 09-install-macos.md, local-postgres-setup.md, local-mongodb-setup.md.

Reference material from the walkthrough moved to the guide that owns it, corrected where it disagreed with the binary: the administration guide gains a command index with every manage subcommand, the assume-role flags, --endpoint, console access and audit, and three runtime settings that were missing; ttl_deletion_target_seconds is not a setting the binary accepts and is no longer listed; control_plane_delay_seconds seeds at 0.25, not 5; throttling is off by default, not on. The deployment guide gains the connection pool formula and worked example; the developer guide gains the external suite runners and the local database setup for the test suite.

Upgrade manual (#495): a container section measured on the published images. Docker Compose from extenddb-postgres 0.1.10 (catalog 0.0.2) to 0.1.14 (catalog 0.0.3): the bootstrap service runs migrate with the server stopped, the table and item written before the upgrade read back unchanged, and a downgrade leaves the catalog alone while the older server refuses to start and restarts under the Compose policy. The one-off form, the preview form and its exit status on the published image, the MongoDB Compose file and the development image are covered. Migration writing moves to a new Contributing page, manuals/14-writing-migrations.md.

Also updated: README, AGENTS.md, the Docker Hub overview source, the installer scripts, the PDF manifest, the sample config comment, the storage backends table, the in-repo skill references, the doc-consistency script's file list, and the site sidebar.

How it was checked

Every command on the development image, Docker Compose, from-source (PostgreSQL) and first-requests pages was run in order against containers and a binary built from this tree; the quoted output on the pages is from those runs. The MongoDB and Cassandra init sections were run against containers when the page was written. Running first-requests in page order found that its transaction example conditioned on an item the Items section had already deleted, so TransactWriteItems was cancelled; the sections are reordered and every block now succeeds in sequence. The shard iterator example carried a literal shard id the CLI rejects client-side; it is a placeholder.

Three reader walks (a backend engineer wanting a server, a platform engineer with PostgreSQL already running, and a cover-to-cover read) completed in one to three clicks each. The rendered site: 66 pages, 5,261 links and every fragment resolved, no removed page in search, no horizontal scroll or clipped code block at 390 or 1280 px in either theme. Strict site build clean at 67 pages; the doc version guard passes.

Two pages this change grows were already over the 600-line mark (admin guide 967 to 1,230; upgrade manual 649 to 734) and are left whole.

Filed separately

• extenddb destroy cannot take Cassandra credentials #500: extenddb destroy has no Cassandra credential inputs, so it cannot tear down a password-authenticated Cassandra deployment.
• Pre-existing items found on pages this change does not touch are filed as issues (differences page default for control_plane_delay_seconds, duplicate headings in the admin and architecture guides, the usage guide's literal shard id, an em dash in the manage create-user warning).

Rebase on main

Rebased with #491 onto main at 1d0d8f4a2. These changes on main touch the pages this PR rewrites, and each is kept:

• docs: document the experimental Cassandra storage backend #412, the Cassandra documentation. Its getting-started text for Cassandra moves to Install from source, where Initialize on Cassandra says the backend is experimental and that the node must run PasswordAuthenticator, then gives the [storage.cassandra] section init writes, the override flags and how to raise the replication factor. The README keeps its list of storage backend prerequisites, with PostgreSQL and MongoDB pointing at Install from source, and AGENTS.md keeps its Cassandra prerequisite, its Cassandra init lines and the Cassandra rows of its documentation table.
• feat(storage): SnapshotEngine, a consistent table snapshot on every backend [backup/restore 4/4] #395. The administration guide keeps its longer control_plane_delay_seconds row, and troubleshooting keeps its table lock step and the section on data tables without a catalog row.
• fix(app): mask secrets in settings list and get #469. The administration guide keeps the paragraph on settings list and settings get masking secret values, ahead of this PR's runtime setting subsections.
• fix(devtools): publish test databases on loopback only #499. The MongoDB container command on Install from source publishes on 127.0.0.1:27017, the change fix(devtools): publish test databases on loopback only #499 made to the local MongoDB page this PR removes.

The deployment guide's two links into the removed MongoDB page, and its pointer to the SQLite section of the getting-started guide, now point at Install from source. With the text from main, the administration guide grows from 988 to 1,251 lines and the upgrade manual from 681 to 768.

Corrections to this PR's own text:

• The Cassandra container on Install from source publishes on 127.0.0.1:9042 only.
• The --cassandra-user and --cassandra-pass bullet agrees with the example above it: the password can come from EXTENDDB_CASSANDRA_PASSWORD, and without --cassandra-user, init connects as $USER, or as cassandra when USER is not set.
• AGENTS.md's Cassandra init line passes the password, the contact point and the superuser as Install from source does, and its list of what init creates names the Cassandra catalog keyspace and the default account's keyspace.

At the rebased head, npm run typecheck and the strict npm run build pass with no broken links or anchors.

CI passed all 24 checks on 560b5bfda. Publish to GitHub Pages is skipped, because it runs only when the event is not a pull request.