Mark documentation as official when built on gh

Without this each doc page is marked as UNOFFICIAL BUILD in the footer.

Signed-off-by: Thomas Frank <[email protected]>

.github/workflows/docs.yml

@@ -36,7 +36,7 @@ jobs:
         sudo apt-get install -y plantuml
 
     - name: Build Sphinx documentation
-      run: make html
+      run: make html OFFICIAL_BUILD=1
 
     - name: Copy redirection index.html
       run: cp github_pages/redirect_index.html build/html/index.html