pre-commit - fix depencency on pydoc-markdown (#5390)

.github/workflows/pre-commit.yml

     - uses: actions/setup-dotnet@v1
       with:
         dotnet-version: '3.1.x'
-    - name: Install dependencies
-      run: |
-          python -m pip install --upgrade pip
-          pip install pydoc-markdown==3.10.1
     - uses: pre-commit/action@v2.0.0
 
   markdown-link-check:

.pre-commit-config.yaml

         entry: utils/validate_release_links.py
     -   id: generate-markdown-docs
         name: generate markdown docs
-        language: script
-        entry: utils/generate_markdown_docs.py --package_dirs ml-agents-envs
+        language: python
+        entry: ./utils/generate_markdown_docs.py --package_dirs ml-agents-envs
+        additional_dependencies: [pyyaml, pydoc-markdown==3.10.1]

utils/generate_markdown_docs.py

     parser.add_argument("--package_dirs", nargs="+")
     args = parser.parse_args()
 
-    ok = False
-    return_code = 0
+    ok = True
     for package_dir in args.package_dirs:
         config_path = os.path.join(os.getcwd(), package_dir, "pydoc-config.yaml")
         print(config_path)
                     subprocess.check_call(subprocess_args, stdout=output_file)
                 remove_trailing_whitespace(output_file_name)
                 new_hash = hash_file(output_file_name)
-                ok = old_hash == new_hash
-
-        return_code = 0 if ok else 1
+                ok &= old_hash == new_hash
-    sys.exit(return_code)
+    sys.exit(0 if ok else 1)