## 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.
* Integrate Python kernel via Binder
* Add live model test for languages with examples
* Update docs and code examples
* Adjust margin (if not bootstrapped)
* Add binder version to global config
* Update terminal and executable code mixins
* Pass attributes through infobox and section
* Hide v-cloak
* Fix example
* Take out model comparison for now
* Add meta text for compat
* Remove chart.js dependency
* Tidy up and simplify JS and port big components over to Vue
* Remove chartjs example
* Add Twitter icon
* Add purple stylesheet option
* Add utility for hand cursor (special cases only)
* Add transition classes
* Add small option for section
* Add thumb object for small round thumbnail images
* Allow unset code block language via "none" value
(workaround to still allow unset language to default to DEFAULT_SYNTAX)
* Pass through attributes
* Add syntax highlighting definitions for Julia, R and Docker
* Add website icon
* Remove user survey from navigation
* Don't hide GitHub icon on small screens
* Make top navigation scrollable on small screens
* Remove old resources page and references to it
* Add Universe
* Add helper functions for better page URL and title
* Update site description
* Increment versions
* Update preview images
* Update mentions of resources
* Fix image
* Fix social images
* Fix problem with cover sizing and floats
* Add divider and move badges into heading
* Add docstrings
* Reference converting section
* Add section on converting word vectors
* Move converting section to custom section and fix formatting
* Remove old fastText example
* Move extensions content to own section
Keep weird ID to not break permalinks for now (we don't want to rewrite URLs if not absolutely necessary)
* Use better component example and add factories section
* Add note on larger model
* Use better example for non-vector
* Remove similarity in context section
Only works via small models with tensors so has always been kind of confusing
* Add note on init-model command
* Fix lightning tour examples and make excutable if possible
* Add spacy train CLI section to train
* Fix formatting and add video
* Fix formatting
* Fix textcat example description (resolves#2246)
* Add dummy file to try resolve conflict
* Delete dummy file
* Tidy up [ci skip]
* Ensure sufficient height of loading container
* Add loading animation to universe
* Update Thebelab build and use better startup message
* Fix asset versioning
* Fix typo [ci skip]
* Add note on project idea label
Always add tooltip text as hidden label. Use different tooltip icons for tags and inline help icons. Add labels to old/new code blocks and add option to customise label text.
