omd2: provide table of contents generation

[avsm]

(noting a missing feature from the omd 1.x branch, that was used in opam2web)

omd 1.x provided a Omd.toc function for table-of-contents generation. The implementation may be re-usable, with the new AST types.

[shonfeder]

Thanks for reporting the omission! I'm considering recovery of this functionality higher priority, but I'd like to settle #223 and #228 first, to minimize rework.

[sonologico]

Started working on this on #240.

[avsm]

Closed by #240 -- it's certainly enough for ocaml.org to generate its ToC. Thanks @sonologico !