docs: automate PRs for new platform version pages

[chrispaterson]

Description

Automates publishing of the Platform Library Reference pages so a new platform
version gets a documentation PR without manual work.

• scripts/sync-platform-versions.py (new) — detects platform versions published to the
  prod archival endpoint (graph.adobe.com/graph/platform/, GRAPH-3604) that have no page
  yet, and downloads each one's @graph/platform-exports bundle. Discovery uses the public,
  unauthenticated major/<n> redirect (latest minor per major) and
  version/<v>/platform-closure.json, walking each major down until it hits an
  already-documented version — so it backfills multiple new minors at once and skips
  historical gaps (e.g. the 2.2 that never shipped). Stdlib only. The version number always
  comes from the endpoint path, never a tarball filename (the exports tarball keeps its
  own package version, e.g. graph-platform-exports-2.11.2.tgz under version/2.17/).
• scripts/gen-platform-pages.py — gains a --version X.Y single-version mode, and
  write_index() now derives the version list from the pages present on disk rather than
  from BUNDLE_DIR. This lets one new version be rendered with a correct, complete index
  without needing every other version's bundle available.
• .github/workflows/check-platform-version.yml (new) — daily cron + manual dispatch:
  sync → generate → lint → open a reviewed PR (peter-evans/create-pull-request).

Motivation and Context

Each new platform release ships a fixed set of importable libraries that plugin authors need
documented. Today that page is produced by hand-running gen-platform-pages.py. This closes
the loop: the docs site polls the same archival endpoint the SDK resolves against, and opens
a ready-to-review PR whenever a new version appears — keeping the reference current with no
standing manual step.

How Has This Been Tested?

Verified end-to-end against the live prod endpoint (graph.adobe.com):

• Reachability — the archival endpoint is a public CloudFront distribution, no auth; the
  version path returns 200 JSON from a non-corp vantage, unknown majors 404, major/<n>
  302-redirects to the latest minor.
• Idempotency — downloading the real 2.17 bundle and running --version 2.17
  reproduces the committed page and index with an empty git diff.
• Add path — a simulated 2.18 produces a new page and moves the — latest marker
  correctly in the index.
• Sync steady state — with everything through 2.17 documented, the sync script prints
  nothing and downloads nothing (workflow no-ops).
• Sync detection — hiding the 2.17 page makes the sync script detect and download exactly
  2.17 (stops at the first already-documented version below it).
• Both scripts py_compile clean.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.