Short and sweet PR that addresses the follow-up in #909: enabling `mermaid` on our docs site, but making it clear that users still need to opt-in to use it. I've also added an example in-action.

To test: [see the Netlify deploy](https://deploy-preview-935--just-the-docs.netlify.app/docs/ui-components/code/#mermaid-diagram-code-blocks).

`mermaid`: refactor config to use `mermaid_config.js` include, only require `mermaid.version` in `_config.yml` (#909)

This PR has a bit of scope creep! This PR now:

- changes the mermaid opt-in logic to only check for the existence of a `mermaid` key instead of `mermaid != false`: this resolves the follow-up in #857
- changes the behaviour of mermaid configuration
    - instead of using `mermaid_init.html` with default settings, makes the include `mermaid_config.js`
    - the include is loaded directly into the contents of `mermaid_initialize`
    - by default, it is an empty object (i.e. `{}`), triggering the defaults
- updates docs
- adds an example to the markdown kitchen sink  

It does significantly change the interface provided in #857, and I apologize for the confusion. However, given the discussion in this PR, I think it's the best move forward!

Closes #825


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>

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

doc for turning on kramdown linenos globally corrected/

Updated native font stack, as suggested in https://github.com/pmarsceill/just-the-docs/issues/330#issuecomment-650415240

For pages in utilities/, reorder `nav_order` to follow `parent`.