Add sphinx-markdown-builder for generating release notes

Without this, Sphinx refuses to use the "-b markdown" builder (see next commit).

doc_src/conf.py

@@ -13,6 +13,14 @@ import sys
 from sphinx.errors import SphinxWarning
 from docutils import nodes
 
+try:
+    import sphinx_markdown_builder
+    extensions = [
+        "sphinx_markdown_builder",
+    ]
+except ImportError:
+    pass
+
 # -- Helper functions --------------------------------------------------------