Add doc generation infra

[wokalski]

We don't have good docs. There's a Makefile with a doc generation script which is a good start. To automatically deploy them to https://dialohq.github.io/ocaml-grpc/ we need to run the doc generation script (preferably in a GitHub action) and then put the result from _build/default/_doc/_html to /docs on the main branch and commit them using the GitHub action user.

It should be as simple as a GitHub workflow like this:

name: Publish docs
on:
  push:
    branches:
      - main 
jobs:
  docs:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - name: Generate docs 
        run: |
          # Run opam install(s)
          make docs
      - name: Commit report
        run: |
          # run only if anything has changed!
          # if $(git diff --quiet) returns a non zero result
          git add docs
          git commit -m "Update docs"
          git push

I wouldn't advise contributors to run the doc generation scripts during development and committing changes because it might introduce some additional conflicts for the generated code that we really don't care about.

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To make the docs nicer we should add a .mld index page