* Add toc heading custom include
Closes#961.
* Revert "Add toc heading custom include"
This reverts commit 49813c341973e313db0a21f075a60ebf2120989e.
* Update code highlighting with line numbers
- Add the example of code highlighting with line numbers explained in the [Jekyll docs](https://jekyllrb.com/docs/liquid/tags/#line-numbers).
- Fix the styling of narrow code with line numbers, which floats to the centre without this CSS adjustment. (The line numbers column expands as needed with larger numbers of lines, despite using `width`; using `min-width` doesn't work.)
To apply HTML compression, I removed `vendor` from `exclude`; that change is left to a different PR.
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 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!
