Add documentation on how to add a highlight.js for additional syntax highlighting support

[DrTobe]

This adds a few lines on how a custom version of highlight.js can be obtained and made available to mdbook. With that information available, all issues about adding support for a specific language should be safe to be closed because mdbook in its default setup probably does not need to support every possible language.

I have seen that there are ongoing efforts to move away from highlight.js but I guess this information may nevertheless be useful in the meantime.

[max-sixty]

Would be good to merge this!

[ehuss]

Unfortunately I don't think just using the highlight.js website will completely work. mdBook is incompatible with newer versions of highlight.js (such as #1622). https://github.com/rust-lang/mdBook/blob/master/CONTRIBUTING.md#updating-highlightjs contains some instructions for building it from source, where you can check out a version from the 10.x series.

[DrTobe]

I just proposed to add this because it took me quite some time to figure this out and it solved our specific problem. So I still think some kind of documentation like this might be valuable but you are right that we should not include it like that if it is not 100% correct. So what specific approach do you propose?

[ehuss]

I'm not sure, but one suggestion might be to copy the first four steps from the contributing guide to this page instead of recommending going to the website.