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>

package/webui/webui.conf

@@ -55,6 +55,15 @@ location = /maintenance/support-bundle {
     include /etc/nginx/webui-proxy.conf;
 }
 
+# On-device User's Guide: the mkdocs site bundled into the rootfs at
+# /var/www/guide by post-build.sh.  Served as static files (no upstream
+# call), public like the published guide.  Present only when the build
+# host had mkdocs; the WebUI hides its User Guide entry when it's absent.
+location /guide/ {
+    alias /var/www/guide/;
+    index index.html;
+}
+
 # Liveness probe — nginx-only, no upstream call. Used by the watchdog
 # div in base.html and the reboot-overlay poller.
 location = /device-status {