* `strings`: Remove unused `hash32_utf8` function
* `strings`: Make `hash_utf8` and `decode_Utf8Str` private
* `strings`: Reorganize private functions
* 'strings': Raise error when non-string/-int types are passed to functions that don't accept them
* `strings`: Add `items()` method, add type hints, remove unused methods, restrict inputs to specific types, reorganize methods
* `Morphology`: Use `StringStore.items()` to enumerate features when pickling
* `test_stringstore`: Update pre-Python 3 tests
* Update `StringStore` docs
* Fix `get_string_id` imports
* Replace redundant test with tests for type checking
* Rename `_retrieve_interned_str`, remove `.get` default arg
* Add `get_string_id` to `strings.pyi`
Remove `mypy` ignore directives from imports of the above
* `strings.pyi`: Replace functions that consume `Union`-typed params with overloads
* `strings.pyi`: Revert some function signatures
* Update `SYMBOLS_BY_INT` lookups and error codes post-merge
* Revert clobbered change introduced in a previous merge
* Remove unnecessary type hint
* Invert tuple order in `StringStore.items()`
* Add test for `StringStore.items()`
* Revert "`Morphology`: Use `StringStore.items()` to enumerate features when pickling"
This reverts commit 1af9510ceb.
* Rename `keys` and `key_map`
* Add `keys()` and `values()`
* Add comment about the inverted key-value semantics in the API
* Fix type hints
* Implement `keys()`, `values()`, `items()` without generators
* Fix type hints, remove unnecessary boxing
* Update docs
* Simplify `keys/values/items()` impl
* `mypy` fix
* Fix error message, doc fixes
* Make serialization methods consistent
exclude keyword argument instead of random named keyword arguments and deprecation handling
* Update docs and add section on serialization fields
<!--- Provide a general summary of your changes in the title. -->
## Description
The new website is implemented using [Gatsby](https://www.gatsbyjs.org) with [Remark](https://github.com/remarkjs/remark) and [MDX](https://mdxjs.com/). This allows authoring content in **straightforward Markdown** without the usual limitations. Standard elements can be overwritten with powerful [React](http://reactjs.org/) components and wherever Markdown syntax isn't enough, JSX components can be used. Hopefully, this update will also make it much easier to contribute to the docs. Once this PR is merged, I'll implement auto-deployment via [Netlify](https://netlify.com) on a specific branch (to avoid building the website on every PR). There's a bunch of other cool stuff that the new setup will allow us to do – including writing front-end tests, service workers, offline support, implementing a search and so on.
This PR also includes various new docs pages and content.
Resolves#3270. Resolves#3222. Resolves#2947. Resolves#2837.
### Types of change
enhancement
## 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.
