CI for documentation

[khieta]

Add CI support for auto-generating documentation.

For reference, how we currently generate documentation is:

1. merge the main branch into the gh-pages branch
2. Run make doc in the gh-pages branch
3. Push the new docs folder

[khieta]

(Reopened & revised description)

[k4rtik]

I may be able to help with this.

[k4rtik]

It seems dune now (as of v3.3) supports creating documentation using the dune build @doc command. See https://github.com/ocaml/dune/releases/tag/3.3.0

I am working on #26, but will let this potential switchover to dune be a later improvement.