Switch to Haiku theme for now.

This is pending getting opendylan-docs working without needing
a submodule which depends on us getting a Sphinx that doesn't have
a bug with using -D html_theme_path=... on the command line:

    https://bitbucket.org/birkenfeld/sphinx/issue/925/
Add notice about sphinx-extensions.
Change any reference to koala to http-server
Use code-block
Merge pull request #19 from fracek/doc_cleanup

Documentation cleanup