Install python3-sphinx-rtd-theme to fix warning

Signed-off-by: Liviu-Mihail Concioiu <liviu.concioiu@gmail.com>

Author: liviuconcioiu

.github/workflows/build.yml

@@ -44,7 +44,7 @@ jobs:
             - name: Install Sphinx-doc
               run: |
                   sudo apt-get update
-                  sudo apt-get install -y sphinx-doc python3-sphinx
+                  sudo apt-get install -y sphinx-doc python3-sphinx python3-sphinx-rtd-theme
             - name: Install gettext
               run: sudo apt-get install -y gettext
             - name: Install git-restore-mtime