ref(docs): silence "not in TOC" warnings for files starting with "_"

Let's start reStructuredText include fragments with underscores as a
convention to help us and Sphinx recognize they aren't standalone
documents.

Author: mboersma

docs/conf.py

@@ -84,7 +84,7 @@
 
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.
-exclude_patterns = ['_build', 'venv']
+exclude_patterns = ['_build', 'venv', '**/_*.rst']
 
 # The reST default role (used for this markup: `text`) to use for all documents.
 #default_role = None