convert to requirements.txt

.github/workflows/photon-api-docs.yml

@@ -82,7 +82,7 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install mkdocs mkdocs-material mkdocstrings[python] mkdocs-gen-files mkdocs-literate-nav mkdocs-section-index mkdocs-exclude
+          pip install -r photon-lib/py/docs/requirements.txt
 
       - name: Build MkDocs site
         run: mkdocs build