Use the "sphinx_rtd_theme" Sphinx theme

.github/workflows/fetch-and-compile-mag-workflow-documentation.yml

@@ -32,11 +32,17 @@ jobs:
         uses: actions/setup-python@v5
         with: { python-version: '3.12' }
       - name: Install dependencies of `metaMAGs` docs
-        run: python -m pip install sphinx
-        # Docs: https://www.mkdocs.org/user-guide/cli/
+        run: python -m pip install sphinx sphinx_rtd_theme
+        # Docs: https://www.sphinx-doc.org/en/master/man/sphinx-build.html
+        #
+        # Notes:
+        # - We use the "spinx_rtd_theme" because that's what the legacy "workflow_documentation" repo uses.
+        # - The `--define` option overrides the corresponding parameter in the `conf.py` file.
+        # - The `--builder` option is an alias for the `-b` option.
+        #
       - name: Compile source documents into HTML
         working-directory: _clones/microbiomedata/metaMAGs
-        run: sphinx-build -b html docs ${{ github.workspace }}/_dist
+        run: sphinx-build --define html_theme='sphinx_rtd_theme' --builder html docs ${{ github.workspace }}/_dist
         # Upload the result as an "artifact" so it can then be downloaded and used by another job.
       - name: Save the HTML for publishing later  # Docs: https://github.com/actions/upload-artifact
         uses: actions/upload-artifact@v4