webui: bundle on-device User's Guide

Build the mkdocs User's Guide into WebUI images and surface it in the UI.
post-build.sh runs `mkdocs build` from the top-level mkdocs.yml into
/var/www/guide when the webui package is selected and mkdocs is on the
build host; nginx serves it read-only at /guide/.  The build is
best-effort — a missing mkdocs warns and ships without the guide rather
than failing — and minimal images (no webui) skip it.

The WebUI gates a book icon in the topbar and a User Guide item in the
user menu on the docs being present on disk (os.Stat, fail-closed), both
opening /guide/ in a new tab.

CI: a local setup-mkdocs composite action installs mkdocs and the
plugins from mkdocs.yml; the build and release workflows run it before
the build so images produced in CI include the guide.  developers-guide
documents the new build dependency and restores the missing
mkdocs-glightbox plugin.

Fixes #633

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

.github/workflows/build-release.yml

@@ -45,6 +45,9 @@ jobs:
           target: ${{ matrix.target }}
           enabled: ${{ inputs.use_cache }}
 
+      # WebUI images bundle the mkdocs User's Guide via post-build.sh.
+      - uses: ./.github/actions/setup-mkdocs
+
       - name: Configure & Build
         env:
           INFIX_RELEASE: ${{ steps.vars.outputs.ver }}

.github/workflows/build.yml

@@ -124,6 +124,9 @@ jobs:
         with:
           target: ${{ env.TARGET }}
 
+      # WebUI images bundle the mkdocs User's Guide via post-build.sh.
+      - uses: ./.github/actions/setup-mkdocs
+
       - name: Configure ${{ env.TARGET }}
         run: |
           make ${{ env.TARGET }}_defconfig