chore: Vendor the UI into this repository

[lfrancke]

Description

The UI moves from the documentation-ui submodule into ui/ in this repository.
When I started this documentation project I just forked the upstream Antora UI and that followed this model. That also made it easy to sync with upstream.

But it's been super annoying to have to do two commits (one in documentation-ui and one in documentation) for each design change. So I decided to vendor it in. Any UI changes are now just one PR 🥳

• This PR should preserve the history as we used git subtree add
• Build extensions moved supplemental-ui/lib/ → lib/: supplemental-ui is the UI file overlay, so the extensions were being published to the live site (/_/lib/*.js). They aren't anymore.
• UI lint/bundle CI moves to .github/workflows/ui.yml, path-filtered on ui/** (the workflow inside ui/.github/ could never trigger from a subdirectory).
• @antora/lunr-extension removed: dead dependency, search is Pagefind.

Verification

Note

This subsection of the PR description was generated by Claude

• Byte-identical build gate: site built from main and from this branch, full diff. Only differences: sitemap <lastmod> timestamps, the intentionally updated sentence on the contributor project-overview page, and the extensions no longer appearing in the published output.
• netlify-fetch tested against a genuine blobless clone (--filter=blob:none, like Netlify's): local release branches created, all release tip trees fully materialized (verified with a non-lazy-fetching check), working tree clean and HEAD untouched afterwards.
• npm run bundle --workspace ui works from the repo root (used by the new CI workflow).

Follow-up after merge

Archive the documentation-ui repository with a README pointer

Reviewer's guide

This PR looks huge, but it's not!

• You can ignore the entire ui subdirectory. It's just copied in
• You can ignore lib as that is also just a moving around thing
• You can ignore the github workflow -> also copied

Basically you can look at the playbooks, makefile, README, ...and that's it

[netlify]

✅ Deploy Preview for stackable-docs ready!

Name	Link
🔨 Latest commit	3822a48
🔍 Latest deploy log	https://app.netlify.com/projects/stackable-docs/deploys/6a577594de9e630008edae76
😎 Deploy Preview	https://deploy-preview-873--stackable-docs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[stackable-cla]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ fhennig
❌ Felix Hennig

---

Felix Hennig seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[lfrancke]

Rebased again: now vendors documentation-ui@d64780c (adds the pagefind asset location hotfix, #152 there) on top of main's newest rate-limiting commits (#880-#882). Clean replay, lint + bundle re-verified.

[xeniape]

Only thing I did was building it locally which seemed to have worked