API documentation

**What would you like to be added**:

Better documentation of the API(s), preferable by auto-generating with e.g. https://github.com/elastic/crd-ref-docs. We'd need to document `api/nfd` and `api/image-compatibility`.

**Why is this needed**:

Better documentation, always up-to-date (if auto-generated)

Review the API definitions under api/nfd and api/image-compatibility, then evaluate the proposed crd-ref-docs generator against them. Done means both APIs have generated, usable documentation that can be kept up to date.

Written by the indexing model from the issue text.