ci: port the Cloudflare Workers docs pipeline to circinus (slug 1.5) (#2208)

* ci: port Cloudflare Workers docs pipeline files to circinus (verbatim)

Copies the branch-agnostic half of the docs.vyos.io Cloudflare Workers
pipeline from `rolling` at 8cb568bf, byte-identical:

  - .github/workflows/docs-build.yml
  - workers/ (entire tree)
  - scripts/docs_gates/
  - docker/im-convert.sh
  - docs/_static/js/version-picker.js, js/pagefind-wrapper.js,
    css/version-picker.css (new files, no circinus counterpart)
  - docs/_templates/breadcrumbs.html, searchbox.html (new files)

docs-build.yml already triggers on push to [rolling, circinus, sagitta]
and resolves `circinus` -> worker vyos-docs-v15-en / slug 1.5 from
workers/matrix.json; the files simply did not exist on this branch, so
slug 1.5 still serves the bootstrap placeholder.

workers/versions.json + workers/matrix.json are deliberately identical
across all three branches and must be kept in sync.

Advances: IS-572

* ci: wire circinus docs build into the Cloudflare Workers pipeline

Hand-merges the CF-specific hunks onto circinus's own docker/Dockerfile
and docs/conf.py rather than clobbering them with rolling's versions —
circinus keeps its own content-driven history in both files.

docker/Dockerfile:
  - imagemagick + librsvg2-bin (sphinx.ext.imgconverter backend) and
    poppler-utils (pdfinfo, used by docs-build.yml's PDF page-count
    completeness check), installed --no-install-recommends
  - install docker/im-convert.sh as /usr/local/bin/im-convert

docs/conf.py:
  - enable sphinx.ext.imgconverter + image_converter = 'im-convert' so
    the LaTeX/PDF builder stops silently dropping .webp/.svg images
  - register js/version-picker.js + css/version-picker.css
    unconditionally (degrades silently on ReadTheDocs)
  - _vyos_cf_build gate off the raw DOCS_VERSION_SLUG env var; only CF
    builds load js/pagefind-wrapper.js, and html_context['vyos_cf_build']
    lets _templates/searchbox.html fall back to the stock Sphinx
    searchbox via the "!" bang-include on RTD

The RTD path stays the default in both files: circinus continues
building on ReadTheDocs until RTD sunset, and every CF feature activates
only when DOCS_VERSION_SLUG is present. .readthedocs.yml is untouched.

circinus keeps its own version/release/html_title/source_suffix and its
hardcoded html_baseurl (its CF slug is also `1.5`, so rolling's
DOCS_VERSION_SLUG/READTHEDOCS_VERSION resolution block is a no-op here
and was deliberately not ported).

Advances: IS-572

* ci: IS-572: re-sync ported Cloudflare Workers pipeline files with rolling

The category-1 files in this port are byte-identical copies from `rolling`.
`rolling` has since moved: [vyos-documentation#2209](https://github.com/vyos/vyos-documentation/pull/2209)
merged as `3a1c6c30`, thirteen rounds of hardening on exactly these files.

Re-take all 14 category-1 paths from `origin/rolling` via
`git checkout origin/rolling -- <paths>`, so byte-identity holds by
construction rather than by hand-editing:

  .github/workflows/docs-build.yml
  scripts/docs_gates/{gates,parity,smoke,test_gates,test_parity,test_smoke}.py
  workers/.gitignore
  workers/apex/src/{index,special,uagate}.ts
  workers/apex/test/{router,uagate}.test.ts
  workers/apex/ua-policy.json

Thirteen of the fourteen carry
[vyos-documentation#2209](https://github.com/vyos/vyos-documentation/pull/2209)
exactly — the pre-change tree was byte-identical to `3a1c6c30^` for those
paths. `workers/.gitignore` additionally picks up the one-line `test-results/`
entry from
[vyos-documentation#2212](https://github.com/vyos/vyos-documentation/pull/2212);
inert on circinus, since only the deliberately-unported `apex-deploy.yml`
writes that directory.

Deliberate exclusions are unchanged: `docs-canary-qa.yml` (cron runs on the
default branch only, so it is not ported even though
[vyos-documentation#2209](https://github.com/vyos/vyos-documentation/pull/2209)
touched it on `rolling`), `apex-deploy.yml`, and the `docs-preview-*`
workflows. `docs/conf.py` stays hand-merged and circinus-specific, with its
ReadTheDocs fallback intact.

🤖 Generated by [robots](https://vyos.io)

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