Consider whether LDoc could be leveraged for developer docs

[alerque]

On a related note to documentation overhauls in #1194 it might be interesting to look at whether LDoc can be leveraged. I would guess this would be more useful for the kind of documentation a package developer might need that a SILE user, but it's possible that better documentation of our own internals could be a benefit.

Two possible outcomes might be a set of developer API docs separate from the user manual using HTML output or writing some sort of wrapper that leverages LDoc to extract all the documentation information then uses SILE itself to typeset this as part of the manual.

[alerque]

We're also going to have Rust API docs. It might be worth looking at tealr too.