docs(extend): retire the merged component docs, redirect to help (PRDCT-543)

[Iamfle4ka]

Jira issue(s): PRDCT-543 — companion delete-from-dev PR for connection-docs#1046 (merged 2026-07-29).

Components, Common Interface, Job Queue and Publish now live at help.keboola.com/extend/** at identical paths. Until this PR lands there are two copies of the same 41 pages, one of which nobody is maintaining.

Changes:

• 41 pages → redirect_to stubs (extend/component 22, extend/common-interface 16, extend/job-queue 1, extend/publish 2). Front matter only: title, the original permalink, the original redirect_from, plus redirect_to: https://help.keboola.com<same path>.
• Stubs rather than deletions. A scan of the keboola GitHub org found 210 files linking to /extend/component/ and 65 to /extend/common-interface/, and that excludes component documentation_urls, the platform UI, support macros and bookmarks. Every redirect_from is preserved, so legacy URLs (/extend/docker/, /extend/docker-runner/, /extend/custom-science/…, /extend/registration/…) keep working — they reach help in two hops.
• 55 links repointed in the 20 pages that stay (Generic Extractor, /integrate/**, /overview/**, /cli/**, index.md, extend/index.md) so they go to help directly instead of through a stub.
• A branded "this page has moved" note — _layouts/redirect.html. jekyll-redirect-from only injects its own template when the site has no redirect layout of its own, so this one file replaces "Redirecting… Click here if you are not redirected." on all 41 stubs at once. The redirect still fires immediately (location.replace + a 0s meta refresh), so nobody waits; the note is what a reader sees when it does not fire, and it asks them to update bookmarks and component documentation_urls. The copy adapts, since this layout renders every redirect_from alias in the repo too: the help wording only for help-bound targets, "the path did not change" only when the from-path and target path actually match (true for the 41 stubs, not for aliases like /extend/docker/ → /extend/component/), and a plain "this page has a new address" for aliases that stay on dev docs. Rendered against those three cases with Liquid in strict mode.
• Nav: the four retired subtrees are gone from _data/navigation.yml.
• /extend/ keeps its landing page for Generic Extractor and Generic Writer (still documented here), with a note pointing at help.keboola.com/extend/ for component development.
• 99 images deleted — every one of them only used by the retired pages, and all already mirrored under public/extend/ on help. component/ui-options/configuration.png stays: Generic Extractor pages still use it.

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Verification:

• All 41 targets and /extend/ itself return 200 on help.
• Every rewritten anchor was checked against the live help page. Three groups needed care:
  • #input-mapping--basic / #output-mapping--basic → #input-mapping-----basic / #output-mapping-----basic. The headings are #### Input mapping --- basic; kramdown turned --- into an em dash, Starlight keeps the literal dashes, so the slug is different. (See the note below — the heading itself is worth fixing on help.)
  • #preparing-the-data-folder → #preparing-data-folder and #creating-a-component → #creating-component: these were already broken on dev, fixed in passing.
• _data/navigation.yml still parses; no reference to a retired path remains anywhere in the repo except the stubs' own front matter.
• Build: not run locally — this repo builds through docker compose and there is no container runtime on my machine, so the Build workflow on this PR is the first build.

Follow-ups, not in this PR:

• Real 301s for developers.keboola.com. redirect_to emits a meta-refresh page (HTTP 200 + canonical); a 301 needs an S3 routing-rule / CloudFront change outside this repo. Given the identity paths, a single /extend/* → help.keboola.com/extend/* rule would cover the whole subtree.
• On help: /extend/common-interface/folders/ links to #output-mapping--headless-csv, which does not exist (same --- heading issue) — a leftover from #1046.

[linear-code]

PRDCT-543

[Iamfle4ka]

Ordering note: the two mapping links in integrate/variables/index.md now point at #input-mapping--basic / #output-mapping--basic, which is what help will generate once connection-docs#1069 lands (it replaces the literal --- in those headings with a real em dash). Merge #1069 first — until then those two anchors resolve to the top of the page rather than the section. Everything else in this PR is independent of it.