convert and move about 1/2 of our docs from .rst to .md

This change starts converting our docs from sphinx-powered-rst to Pants-enhanced-md, moving files to some different directories:

src/docs/*.md  <= language-agnostic Pants-user docs

examples/src/java/com/pants/examples/*.md  <= Pants-with-Java docs

examples/src/python/example/*.md  <= Pants-with-py docs

src/python/pants/docs  <= good place for pants-dev docs

Keeping foolang docs with examples/src/foolang feels right; if we change the examples, we probably want to change the docs, too. Less-certain about where language-agnostic docs go.

Testing Done:
eyeball-compare pages gen'd from .rst, .md

Reviewed at https://rbcommons.com/s/twitter/r/1176/
Sync of https://github.com/pantsbuild/pants/commit/5d923a1afb5c653081cc3c9fc181bfe4c8e46ab5