💫 Update website

[ines]

Description

The new website is implemented using Gatsby with Remark and MDX. This allows authoring content in straightforward Markdown without the usual limitations. Standard elements can be overwritten with powerful React 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 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

• I have submitted the spaCy Contributor Agreement.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.