rf-rajj: modernise mkdocs-material 5.5.9 → 9.x

CI image bump:
  .github/workflows/docs-workflow.yml — squidfunk/mkdocs-material:5.5.9 → :9.
  Following the bead's Phase 5 plan, will re-pin to a specific 9.x.y tag once
  this lands cleanly on master.

mkdocs.yml migrations:
  theme.features:
    - tabs                  → navigation.tabs
    - (added navigation.footer to compensate for the deleted footer-partial
      override; the prev/next pagination is now Material's default behaviour
      gated on this feature)
  theme.hljs_languages:
    - removed; Material 9 uses pymdownx.superfences + pygments and detects
      Clojure automatically from ```clojure fenced blocks. Verified by
      build: site/subscriptions/index.html has highlight-class hits on
      every Clojure block.

Custom-dir audit (the careful part):
  Two of the six theme override files were unmodified verbatim copies of
  Material 5's partials, with no project-specific customisation. Their HTML
  used Material 5's BEM names (md-header-nav__*, md-footer-nav__*) which
  Material 9 renamed to md-header__* / md-footer__*. Keeping them under
  Material 9 would have rendered the header and footer with class names that
  no Material 9 CSS rule targets — silent visual breakage with no build
  error.

  Deleted (no customisation, stale class names):
    docs/theme/partials/header.html
    docs/theme/partials/footer.html

  Kept (real customisations, blocks still valid in Material 9):
    docs/theme/main.html              — block htmltitle (no " - site_name"
                                        suffix), block footer (loads docs.js)
    docs/theme/404.html               — custom 404 content (block content)
    docs/theme/overrides/main.html    — OG/Twitter meta (block extrahead)
    docs/theme/overrides/home.html    — homepage hero (block tabs)

  Side-effect of deleting the partials: we now also gain palette-toggle
  hooks, alternate-language switcher, sticky-tabs support, and modern
  search styling that the old verbatim copies were silently masking.

Verified locally with docker run --rm -v "\$PWD":/docs squidfunk/mkdocs-material:9 build:
  - 0 deprecation warnings on the migrated config
  - 0 template-resolution errors on the trimmed custom_dir
  - md-header__* and md-footer__* classes present in rendered HTML
  - md-tabs nav element rendered (navigation.tabs working)
  - md-footer__link--prev / md-footer__link--next rendered
    (navigation.footer working)
  - Clojure code blocks highlight correctly without hljs_languages
  - Custom 404 (not-found-container, 404-img.png) survived block content
  - 5 pre-existing warnings (releases/2016.md broken links, README/index
    conflict) unchanged from the Material 5 baseline; not migration-induced

Phase 4 visual-review gate per the bead is operator-owned; the build
artifact is in /docs/site for inspection. After visual approval, Phase 5
is to re-pin :9 to a specific 9.x.y tag.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mike-thompson-day8

.github/workflows/docs-workflow.yml

@@ -81,7 +81,7 @@ jobs:
       - re-frame-docs-app
     runs-on: ubuntu-24.04
     container:
-      image: "squidfunk/mkdocs-material:5.5.9"
+      image: "squidfunk/mkdocs-material:9"
     steps:
       - name: Checkout Current Branch
         uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4

docs/theme/partials/footer.html

@@ -22,11 +22,10 @@ theme:
     text: IBM Plex Sans
     code: Source Code Pro
   features:
-    - tabs
+    - navigation.tabs
+    - navigation.footer
     - content.code.annotate
     - content.code.copy
-  hljs_languages:
-    - clojure
 
 extra:
   social: