## 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.
* issue_2385 add tests for iob_to_biluo converter function
* issue_2385 fix and modify iob_to_biluo function to accept either iob or biluo tags in cli.converter
* issue_2385 add test to fix b char bug
* add contributor agreement
* fill contributor agreement
## Description
Fix for issue #2361 :
replace &, <, >, " with &amp; , &lt; , &gt; , &quot; in before rendering svg
## 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.
- [ ] I ran the tests, and all new and existing tests passed.
(As discussed in the comments to #2361)
- [x] My changes don't require a change to the documentation, or if they do, I've added all required information.
* Simplify is_config() and normalize_string_keys()
* Use __in__ to avoid the nested _ands_ and _ors_.
* Dict comprehension directly tracks with the doc string
* Keep more basic loop in normalize_string_keys
* Whitespace
* Add logic to filter out warning IDs via environment variable
Usage: SPACY_WARNING_EXCLUDE=W001,W007
* Add warnings for empty vectors
* Add warning if no word vectors are used in .similarity methods
For example, if only tensors are available in small models – should hopefully clear up some confusion around this
* Capture warnings in tests
* Rename SPACY_WARNING_EXCLUDE to SPACY_WARNING_IGNORE
* Go back to using requests instead of urllib (closes#2320)
Fewer dependencies are good, but this one was simply causing too many other problems around SSL verification and Python 2/3 compatibility. requests is a popular enough package that it's okay for spaCy to depend on it – and this will hopefully make model downloads less flakey.
* Only download model if not installed (see #1456)
Use #egg=model==version to allow pip to check for existing installations. The download is only started if no installation matching the package/version is found. Fixes a long-standing inconvenience.
* Pass additional options to pip when installing model (resolves#1456)
Treat all additional arguments passed to the download command as pip options to allow user to customise the command. For example:
python -m spacy download en --user
* Add CLI option to enable installing model package dependencies
* Revert "Add CLI option to enable installing model package dependencies"
This reverts commit 9336ffe695.
* Update documentation
