docs: align shared demo design and rollout with deployed backend

[suguanYang]

Summary

We update the demo documentation to describe the deployed shared corpus rather than the superseded personal-reference proposal. The docs record the dated production restoration snapshot (27 READY sources, 2,562 chunks, generation 27), correct table names and runtime authorization, and document v2 parser routing, catalog discovery, revision-aware reads, client migration, and rollback.

We mark the old reference design as historical, remove broken links, correct the runtime-role preflight query, and distinguish the local SpaceX coverage result from the production snapshot (388 of 407 pages indexed). This follows backend PR #451.

Verification

• git diff --check origin/main...HEAD
• All relative links in the three changed documents resolve in an isolated checkout of the latest main plus this commit.
• Credential scan passed; only the three demo Markdown documents are included.
• Table names and transaction-local maintainer authorization were checked against implementation. Production figures are from retained 2026-10-05 evidence; no live cloud checks or runtime tests were repeated for this documentation change.

Deployment Notes

This PR changes documentation only and requires no deployment or migration. The documented backend contract removes materialization (410) and requires clients to use catalog-provided shared IDs and revisions. Notebook migration remains a separate follow-up. Rollback guidance retains additive tables and assets.

Checklist

• Public docs and examples were updated
• Logs, errors, and validation paths avoid leaking secrets or user data
• The pull request description explains breaking or user-visible changes
• Tests and migrations: not applicable to this documentation-only change