Though homepage is rendered completely from a template,
it still needs an index.md to generate.
Also change subpage breadcrumb links to relative, so they
work in the GitHub Pages context. For example "/" points
to the Google Org's home rather than Draco's home, so use
"..", etc.
For building docs locally, there's no need to specify
an alternate `--baseurl` as long as `jekyll serve`` is
issued from the `/docs` directory, as indicated in
README.md.
Removed --baseurl guidance and emphasized README.md
instructions.
