Tweak sphinx implementation

This matches what I'm doing on other libraries. Moves sphinx-src to docs
and uses docs/_build instead of sphinx-build. Also adds the sphinx
default Makefile in docs for building html etc. My pattern is to run
`aspen -w _build/html &` and then do `make html` over and over again.
Not ideal but it works and it's what I'm used to and I WANT TO WRITE
DOCS AND RIP OUT SERVER>PY!!!!!!!!!!! 8D