Documentation about usage and expected documentation

AFAIK the documentation specification is currently held in the transformers' documentation guide [here](https://github.com/huggingface/transformers/tree/master/docs#writing-documentation---specification). Having it visible in `doc-builder` with everything that was learned when doing the port of `transformers`, `datasets`, `accelerate`, `optimum` and `huggingface_hub` would be super nice.

Additionally, a breakdown of everything that needs to be done to port a library would help (linking to PRs that did would also help!)., see https://github.com/huggingface/huggingface_hub/issues/759