In touching up the migration guide, I noticed that many of our documentation site links are broken! For example, on the homepage, this link:

<img width="782" alt="screenshot of homepage; code snippet is in next block" src="https://user-images.githubusercontent.com/14893287/210462690-31aa7bf5-dd79-4e8f-a3c5-1213e73771c4.png">

which has the following href

```code
<a href="/just-the-docs/just-the-docs/CHANGELOG/">the CHANGELOG</a>
```

duplicates the `baseurl` twice. There are 14 such broken links across the site. Each link duplicates the `baseurl` and `link` tags, which has since been resolved with links being relative by default (there's a set of PRs that document this - I can't find the exact paper trail right now).

To resolve this, I:

- find and replace site-wide `{{ site.baseurl }}{% link` with `{% link`
- tested each link, which now works properly locally *and* on the deploy preview

I'm surprised we didn't catch this earlier! I also could be missing something else, in which case feedback on this PR is certainly welcome.

* 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.

* Make CHANGELOG urls clickable

- Add link reference definitions for PRs and new contributors in v0.4.0.rc2
- Replace inline urls by shortcut link references
- Add front matter so the page appears in the navigation
- Change the link to the CHANGELOG on the home page

The CHANGELOG contents should look the same in the docs and in the repo (apart from the callouts) with clickable links to PRs and some contributors.

The urls shown in the changes in v0.4.0.rc1 (and previous releases) haven't yet been made clickable – it would be too tedious and error-prone to do it manually.

* Make CHANGELOG urls clickable

- Add a callout

prep for v0.4.0.rc2

This PR updates the home page and the CHANGELOG to refer to  v0.4.0.rc1 as a pre-release or release candidate, rather than a release. See [this comment](https://github.com/just-the-docs/just-the-docs/pull/613#issuecomment-1240442518) for motivation.

It also adds the versioned docs issue (#728) to the roadmap in the CHANGELOG.

As the config for the theme docs now needs to declare callouts, the [callouts docs](https://just-the-docs.github.io/just-the-docs/docs/ui-components/callouts/) can now illustrate the rendered appearance. (These callouts are merely examples: the names and colors should eventually be replaced by a principled collection, taking account of WCAG.)

Co-authored-by: Matt Wang <matt@matthewwang.me>

Co-authored-by: Matt Wang <matt@matthewwang.me>

Co-authored-by: Matt Wang <matt@matthewwang.me>