API documentation

[sim642]

While #208 is about documenting high-level usage and development patterns, the OCaml Goblint library also has ocamldoc comments here-and-there (and should obviously have more!) but there's no good documentation for the codebase itself.
Such documentation would also cover some more implementation-specific points from #208.

TODO

• GitHub Actions job to publish odoc to GitHub pages, as is standard for OCaml packages.

• Overview/structure of the OCaml codebase.

  There is https://github.com/goblint/analyzer/blob/6a41ac5f84fa561d100139e82cbbe28d3461a898/src/main.camldoc, but it is very outdated, unused, incomplete and incompatible with odoc (due to {!indexlist}).