install sphinx deps from docs/requirements.txt file

instead of keeping the listed deps in sync across different repos

.github/workflows/sphinx.yml

@@ -18,7 +18,7 @@ jobs:
         with:
           python-version: 3.x
       - name: Install dependencies
-        run: pip install sphinx-immaterial -e .
+        run: pip install -r docs/requirements.txt -e .
 
       - name: Build docs
         run: sphinx-build docs ${{ inputs.path-to-doc }}