Enable sphinx.ext.doctest, make other fixes

In `docs/conf.py`, enable the "doctest" extension. This allows the `make
doctest` command to be used. Rewrite `docs/getting_started.rst` to make use of
the extension.

Make git ignore documentation build artifacts.

Comment out the unused `html_static_path` option from `docs/conf.py`. It was
causing a warning when `make html` was executed.

Clean up some other minor stuff. Properly wrap lines at 80 chars wide, remove
some spurious blank lines, remove trailing whitespace.