Improve syntax highlight for short function signature block on api docs

From @sprunk:
> Syntax coloring is somewhat poor:
>
> * the `nil` return is colored but the `boolean` or `string` returns are not (I guess it's because it's implicit? Shouldn't matter though)
> * most words are still white, harder to tell the function name and the args apart
> * the `?` is a bit hard to see. This is on light mode, it gets a bit better with dark mode
>
>
>

Note currently we use the `lua` syntax highlighting for the "short signature" codeblock, we're free to devise the format of the block and the syntax highlighting language as we want to improve this. See [templating](https://github.com/beyond-all-reason/RecoilEngine/blob/9edb166/doc/site/docgen/templates.rb#L51-L54)

Ideas:

- Consider trying a different language codeblock, e.g. `typescript`?

Start in doc/site/docgen/templates.rb at the short-signature template and inspect how its Lua code block is generated. Compare the current highlighting with the suggested TypeScript alternative, then verify that return types, function names, arguments, and the question mark are distinguishable in light mode. Done means the API short-signature block has clearer syntax coloring.

Written by the indexing model from the issue text.