ci: Build documentation with hotdoc to verify Since markers

We use GStreamer image to build the documentation as it is the simplest
way to ensure that we are testing against the same thing as GStreamer
itself.

.gitlab-ci.yml

@@ -10,6 +10,12 @@ include:
 variables:
   FDO_UPSTREAM_REPO: gstreamer/gst-plugins-rs
 
+  # We use GStreamer image to build the documentation as it is the simplest way
+  # to ensure that we are testing against the same thing as GStreamer itself.
+  #
+  # This shouldn't be updated so often, only when big updates happen in
+  # GStreamer itself
+  GSTREAMER_DOC_IMAGE: registry.freedesktop.org/gstreamer/gstreamer/amd64/fedora:2022-09-14.0-main
   WINDOWS_BASE: "registry.freedesktop.org/gstreamer/gstreamer-rs/windows"
   WINDOWS_RUST_MINIMUM_IMAGE: "$WINDOWS_BASE:$GST_RS_IMG_TAG-main-$GST_RS_MSRV"
   WINDOWS_RUST_STABLE_IMAGE: "$WINDOWS_BASE:$GST_RS_IMG_TAG-main-$GST_RS_STABLE"
@@ -233,6 +239,48 @@ meson static:
     paths:
     - 'build/meson-logs/'
 
+# Check that the gstreamer documentation keeps working
+documentation:
+  image: $GSTREAMER_DOC_IMAGE
+  variables:
+    MESON_ARGS: >
+      -Domx=disabled
+      -Dpython=disabled
+      -Dlibav=disabled
+      -Dlibnice=disabled
+      -Dugly=disabled
+      -Dbad=disabled
+      -Dgood=disabled
+      -Ddevtools=disabled
+      -Dges=disabled
+      -Drtsp_server=disabled
+      -Dvaapi=disabled
+      -Dsharp=disabled
+      -Dgst-examples=disabled
+      -Drs=enabled
+      -Dgst-plugins-rs:sodium=system
+      -Dgst-docs:fatal_warnings=true
+      -Ddoc=enabled
+      -Dbase=disabled
+      -Dbase=disabled
+      -Dorc=disabled
+  script:
+    - P=$(pwd)
+    - cd ..
+    - rm -rf gstreamer
+    - git clone --depth 1 https://gitlab.freedesktop.org/gstreamer/gstreamer.git --branch main
+    - cd gstreamer
+    - ln -s $P subprojects/gst-plugins-rs
+    - meson build $MESON_ARGS
+    - ./gst-env.py ninja -C build subprojects/gst-docs/GStreamer-doc
+    - mv build/subprojects/gst-docs/GStreamer-doc/html documentation/
+  artifacts:
+    expire_in: '7 days'
+    when: always
+    paths:
+      - documentation/
+  needs: []
+
 # build gst-plugins-rs as a gst-build subproject
 gst-build:
   extends: .meson