* Update the theme home page

Major update:
- Refer to `just-the-docs-template` for getting started.
- Avoid duplication of details provided in the template docs.
- Distinguish between theme users and theme developers.
- Update the callouts.
- Make the marked-up text easier to read and edit by using short-cut link references.

* Update index.md

- Add a reference to the theme README for using the theme as a gem.
- Add a footnote separator line.