make odoc does not generate links to every module

[fblanqui]

For instance _build/default/_doc/_html/lambdapi/Core/index.html contains no link to the modules Ctxt, Env, etc. although the files are generated.

Note also that CI does not check that make odoc succeeds with no warnings.

It would be nice to improve the rendering of those files too.

Useful link: https://ocaml.github.io/odoc/odoc/odoc_for_authors.html#interfaces