GeezerVison

Summary

Moves four legacy redirects into release/2.39's docs/redirects.json, so they apply only while 2.39 is the docs mainline:

Today the same rules live in coder.com's and docs-engine's static redirect files, which apply whatever the mainline is. On main, #31645 moves Templates back to docs/templates/ and adds admin/templates/* → templates/*, so once 2.40 is the mainline the static rules loop. coder/coder.com#1201 and coder/docs-engine#304 remove them, but on their own they'd leave these URLs returning 404 on 2.39 until 2.40 ships.

With the rules here instead:

• coder.com reads the mainline branch's docs/redirects.json for unversioned /docs, so the four URLs keep redirecting while 2.39 is mainline and stop when 2.40 takes over, with no merge to time.
• The router applies a version's own rules on a miss, only when the destination page exists, so /docs/v2.39/templates keeps redirecting after 2.40 ships too.

Merge order

1. This PR. It's safe alone: coder.com's static rules run first and point to the same places.
2. After coder.com rebuilds with this commit pinned and docs-engine republishes 2.39, confirm /docs/templates still lands on /docs/admin/templates.
3. Merge coder/coder.com#1201 and coder/docs-engine#304.

Testing

• go run ./scripts/docsredirectscheck on this branch: docs/redirects.json: 5 rules OK. Destinations are pages in 2.39's manifest, no source is a live page, and there are no chains.
• docs-engine's parseVersionRedirectsFile, the publisher's parser, accepts the file and normalizes all five rules.

Refs DOCS-1479.