PRDCT-583: redirect the migrated /extend/ pages to help (dev-side delete for #1046)

[Iamfle4ka]

Second half of the Extending Keboola merge unit. Component building, Common Interface, Job Queue and Publish moved to help.keboola.com in connection-docs#1046 (merged 2026-07-29); this stops dev from serving a second copy of all 41 pages.

Per the 2026-07-27 navigation call the shape is two PRs per unit — merge into help first, then delete from dev. This is the delete half, and it is the first one for this unit.

What changes

• 41 pages under extend/common-interface/ (16), extend/component/ (22), extend/job-queue/ (1) and extend/publish/ (2) become redirect_to stubs. 8177 lines of duplicated content go away.
• permalink is unchanged on every page, so each /extend/... URL keeps working and now lands on its help twin instead of a stale duplicate.
• The 12 pages that carried redirect_from aliases keep them verbatim — old URLs like /extend/custom-science/php/ still resolve.
• _data/navigation.yml — the four subtrees removed: 168 url entries → 127, exactly the 41 in scope.

jekyll-redirect-from is already in _config.yml, so no new dependency.

Deliberately not included

• extend/index.md stays a real page. It is what still links the two unmigrated subtrees together on dev, and its body points at /extend/generic-extractor/**.
• extend/generic-extractor/** and extend/generic-writer/** stay untouched — their help-side PRs (#1054, #1052) are still open. Redirecting them now would send readers to pages that do not exist yet.
• Image assets (~99 files) are left in place. They are unreferenced once the pages are stubs, but deleting binaries is easy to do later and impossible to undo cheaply if something external hotlinks them.

Verified

• Every one of the 41 redirect targets exists on main in connection-docs — checked against the actual page list, not assumed.
• Nav arithmetic checked both ways: the four in-scope subtrees are gone, /extend/, generic-extractor (38 entries) and generic-writer (3) are untouched, and the YAML still parses.

🤖 Generated with Claude Code

[linear-code]

PRDCT-583

[Iamfle4ka]

Closing as a duplicate — #406 covers the same 41 pages and goes further: it repoints 55 inbound links across the 20 pages that stay, validates every rewritten anchor against the live help HTML (catching three real mismatches), deletes the 99 now-orphaned images after checking each is mirrored on help, and keeps /extend/ as a landing page with a pointer to where the component docs went. This PR did none of that. #406 is the one to review.