mention syntax highlight requirements in track documentation

[lucaferranti]

Currently, exercism needs

• a codemirror plugin (for the editor)
• a highlightjs plugin (for the rest)

As far as I can tell, these requirements are not mentioned in the track documentation, it would be very nice to have at least a short mention about these requirements, so that people who start working on a new track can know it from the beginning.

[ee7]

Sounds good to me to improve docs regarding highlightjs and codemirror. Currently there's a little mention regarding highlightjs:

docs/building/tracks/new/prepare-for-launch.md

Line 54 in 10e5720

- `highlightjs_language`: the language identifier for Highlight.js (see the [full list of identifiers](https://github.com/highlightjs/highlight.js/blob/main/SUPPORTED_LANGUAGES.md)). If there is no Highlight.js support, this field can be omitted.

However, I think Exercism staff usually handle this when launching a track - for codemirror, the required change is in the exercism/website repo, which is mostly for staff.

I think it's also not strictly a requirement to have those plugins - it's just a requirement for syntax highlighting. There wasn't a codemirror plugin for Zig, so the track initially launched without syntax highlighting. And we can always just pick another language as a workaround (Zig is highlighted as Rust).