This is an alternative PR that resolves #1011. Unlike #1013, this PR defines a *new* SASS file, `_sass/custom/setup.scss`, specifically designed for new custom variables (and other SASS-only constructs). It's imported after our `support` SASS files are (functions, variables), but otherwise is imported before all other files (ex, when CSS is emitted).

So, custom callout colors can now be defined in this file. I also move the custom callout colors present in `custom.scss` to the right location.

I've added some docs that briefly explain how to use the feature. Feedback is welcome!

---

As an aside, I chose not to add a `_includes/css` file that imports this, and then import that file. I think that's only necessary if we're trying to render liquid somehow in the SASS file; since we're not trying to do that for `setup.scss`, I've opted to not include it. If we think this is relevant, I can re-add it.

Co-authored-by: Peter Mosses <18308236+pdmosses@users.noreply.github.com>

This way, custom styles are able to override the callout styles, which were previously included last.

* Fix default highlighting in custom schemes

Fix #982

The variable settings for highlighting in the default `light` scheme are currently (v0.4.0.rc2`) in `_sass/color_schemes/light.scss`.
This PR ensures that custom schemes are based on the `light` scheme.

It also adds a note explaining the default to the customization docs,
and gives an example of how to define a custom scheme based on the `dark` scheme

* Prettier

* Deleted test file

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

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>

(cherry picked from commit 7fb79c2018b979fdf917f427ca5d3f2451f3265d)

(cherry picked from commit 8aa0899facd01f689692fccdb338673f5e9a9fbd)

(cherry picked from commit bab3437a1feefc49213e1e787db0451491aca9b9)

(cherry picked from commit 52b0bced5d235d7557413e064f3841ea774d9063)