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

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