Add documentation infrastructure

[lucic71]

The documentation will sit in the docs/ dir which contains a mdbook. The new Deploy docs workflow (inspired from here) builds the mdbook and, using Github Pages, hosts it at https://cpp2rust.github.io/cpp2rust/

@nunoplopes before doing the merge, do the following steps to enable Github Pages:

1. Go to Settings -> Pages -> Build and deployment -> Source -> Select "GitHub Actions"
2. Merge the PR