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.
docs(quick-start): add "Get the Source" instructions
Merge pull request #2513 from mboersma/git-and-gopath

docs(quick-start): add "Get the Source" instructions