## Description
This PR adds the most relevant documentation of spaCy's Cython API.
(Todo for when we publish this: rewrite `/api/#section-cython` and `/api/#cython` to `/api/cython#conventions`.)
### Types of change
docs
## Checklist
<!--- Before you submit the PR, go over this checklist and make sure you can
tick off all the boxes. [] -> [x] -->
- [x] I have submitted the spaCy Contributor Agreement.
- [x] I ran the tests, and all new and existing tests passed.
- [x] My changes don't require a change to the documentation, or if they do, I've added all required information.
