In #1058, I noted:

> Tangentially related work:
> ...
> - better annotate new features (motivated by writing these docs)
>     - we should add "New" to new features :) 
>     - we should note when a feature was introduced (I think this is a core part of most software documentation)
>     - we should annotate things that are "Advanced" in so far as the average Just the Docs user will not use them / they require significant Jekyll knowledge
> 

This came up again in https://github.com/just-the-docs/just-the-docs/discussions/1136#discussioncomment-4716253, so I think it's best for us to resolve this sooner rather than later.

This PR is me doing that. I:

- have added a headings-level "New" label to every new heading introduced since `v0.3`
- added, when possible, inline YAML comments when new configuration options have been introduced

I did this by scanning through the CHANGELOG and selecting each feature that is either tagged with `Add` and has documentation.

I may have also missed any new features, so some double-checking would be helpful!

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.

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.)

This PR combines (and resolves conflicts between) #448, #463, #466, #494, #495, #496, #498, and #572. 

The main aim is to facilitate use of several of the implemented features _together_, when using the fork as a remote theme. It should also simplify merging the included PRs into a future release.

The branch [combination-rec-nav](https://github.com/pdmosses/just-the-docs/tree/combination-rec-nav) adds [multi-level navigation](https://github.com/pmarsceill/just-the-docs/pull/462) and (NEW:) [sibling links](https://github.com/pmarsceill/just-the-docs/pull/394) to the branch used for this PR. It includes updated [documentation for the navigation structure](https://pdmosses.github.io/just-the-docs/docs/navigation-structure/), and reorganised and extended [navigation tests](https://pdmosses.github.io/just-the-docs/tests/navigation/). The documentation and the tests can be browsed at the (temporary) [website published from the combination-rec-nav branch](https://pdmosses.github.io/just-the-docs/

).

_Caveat:_ The changes to v0.3.3 in this PR and #462 have not yet been reviewed or approved, and may need updating before merging into a release of the theme. If you use a branch from a PR as a remote theme, there is a risk of such updates affecting your website. Moreover, these branches are likely to be deleted after they have been merged. To avoid  such problems, you could copy the branch that you want to use to your own fork of the theme.

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