mpg: storage is billed on usage

[miguel-fly]

Summary of changes

• /postgres#pricing: storage is billed on usage, and v1 clusters are noted as still billed on their provisioned storage
• /postgres/create-and-connect: the 10GB default is an initial volume size that grows automatically with the data
• /about/pricing: MPG pricing depends on the plan and the storage the databases use

Preview

• https://fly-io-mpg-usage-billing-docs.mintlify.site/about/pricing
• https://fly-io-mpg-usage-billing-docs.mintlify.site/postgres#pricing

Notes

• Capacity facts (1 TB maximum, up to 500 GB at creation) are unchanged; this PR only touches billing language.
• Rebuilt on the MDX tree after the Mintlify cutover, same wording as the original commit.

[kcmartin]

This can't merge as it stands. It edits the old .html.md files, and those went away at the Mintlify cutover. The pages are MDX now.

I can port it today if you want, it's a small change. One thing: mpg/overview is being retired, so that part would get dropped. And MPG is moving from /mpg to /postgres in #2503, so sooner is easier.

Tracking the stranded PRs in superfly/docs-tracking#314.

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[miguel-fly]

@kcmartin I've updated the PR. Can you please take a look again? I think it's ready for merging

[kcmartin]

Thanks for rebuilding this on MDX. It merges cleanly onto main now, Vale passes and the preview looks right. Two small things inline, then I am happy to merge.

[kcmartin]

Two things that are not yours to fix, noting them here so they do not get lost.

The flyctl help block in create-and-connect is already out of date independently of this PR. It is missing --enable-postgis-support, --pg-major-version and -h, and its plan list reads (basic, launch, scale) while flyctl now prints Basic, Starter, Launch, Scale, Performance. The pricing table on /postgres has all five, so the table is right and the copied block is wrong. Vincent patched the same block last week in #2501 when he removed --pgvector. Two people hand-patching a copy in a fortnight is the argument for replacing it with a link to the generated page, which is tracked in superfly/docs-tracking#302.

And a question for whoever owns MPG billing, which I cannot answer from the docs: is storage metered continuously or on peak? The example here assumes a flat 10GB for the month. If a cluster grows from 10GB to 100GB on the 15th, the page does not say what that costs. Worth settling before this lands, since it is a pricing page and it will be the first thing someone asks.

[kcmartin]

LGTM! Thanks for all your work on this one!