Docs: use the sphinx-rtd-theme with support for addons integration

Installs a version of our theme from a pull request that has support for the
Addons integration using the `CustomEvent`.

Related: readthedocs/sphinx_rtd_theme#1526
Requires: #11205

.readthedocs.yml

@@ -31,6 +31,12 @@ build:
           exit 183;
         fi
 
+    # Install our Sphinx Theme version that uses the Addons CustomEvent to generate the integrated flyout
+    # TODO: this custom installation once a new release has been done
+    post_install:
+      - pip uninstall sphinx-rtd-theme
+      - pip install git+https://github.com/readthedocs/sphinx_rtd_theme.git@humitos/addons-flyout#egg=sphinx-rtd-theme
+
 search:
   ranking:
     # Deprecated content