feat(developer): redirect legacy docs URLs to guides

[kevincador]

Map legacy /docs/* URLs to their corresponding developer portal guides after the hostname redirect preserves the original path.

For example, /docs/caching-and-fresh-metadata opens /?section=guides&guide=caching-and-fresh-data.

Covers 24 docs pages and two aliases, including trailing slashes and .md URLs. Unknown paths fall back to the homepage. Reference pages have no dedicated mappings.

The shim runs in the browser through the existing GitHub Pages 404 fallback. The hostname redirect must be configured separately.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-24T08:21:42.138868Z	a98bb3b	PR opened

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review" or "@codex security review".

Codex reacts with 👀 while any review is running, comments if it has suggestions, and reacts with 👍 once all reviews finish with no findings.

[vladjerca]

Checked out and ran the portal gate: deno fmt --check, prettier check, deno task check (0 errors with the PUBLIC_* vars set), deno task test (46 files, 250 tests) and deno task build all pass. The build emits the 404.html fallback the shim relies on.

The spec covers all 24 guides against contents.md, both aliases, trailing slash and .md variants, and the unknown-path fallback (including /docs/constructor, which the Map lookup handles). Nothing to fold, so I left the branch untouched.

Reminder from the description: the hostname redirect still needs configuring separately, otherwise the shim never sees the old URLs.