Utilities/Sphinx: Add option to build outdated version banner for cmake.org

This was previously added in a custom branch for building the
cmake.org reference documentation.

.gitlab/os-linux.yml

@@ -492,4 +492,5 @@
         - cd build/
         - cmake ../Utilities/Sphinx -GNinja -DSPHINX_HTML=ON
           -DCMake_SPHINX_CMAKE_ORG=ON
+          -DCMake_SPHINX_CMAKE_ORG_OUTDATED=$CMAKE_CI_SPHINX_OUTDATED
         - ninja

Utilities/Sphinx/CMakeLists.txt

@@ -141,6 +141,10 @@ if(CMake_SPHINX_CMAKE_ORG)
     -A googleanalytics=1
     -A versionswitch=1
     )
+
+  if(CMake_SPHINX_CMAKE_ORG_OUTDATED)
+    list(APPEND doc_html_opts -A outdated=1)
+  endif()
 endif()
 
 set(doc_format_outputs "")

Utilities/Sphinx/static/cmake.css

@@ -22,3 +22,15 @@ div.sphinxsidebarwrapper {
 dd > :first-child > p {
   margin-top: 0px;
 }
+
+div.outdated {
+  background-color: #f0f0c0;
+  color: black;
+  font-size: 90%;
+  padding-bottom: 5px;
+  padding-left: 2px;
+  padding-right: 2px;
+  padding-top: 5px;
+  text-align: center;
+  width: 100%;
+}

Utilities/Sphinx/templates/layout.html

@@ -17,6 +17,19 @@
   </li>
 {% endblock %}
 
+{%- block relbar1 %}
+{{ super() }}
+{%- if outdated is defined %}
+    <div class="outdated">
+      This documents an old version of CMake.
+      <a href="https://cmake.org/cmake/help/latest/{{ pagename }}.html">
+        Click here to see the latest release.
+      </a>
+      <span class="version_switch_note"></span>
+    </div>
+{%- endif %}
+{%- endblock %}
+
 {% block extrahead %}
   {%- if versionswitch is defined %}
     <script type="text/javascript" src="{{ pathto('../version_switch.js', 1) }}"></script>