Add Docusaurus site with current documentation and blog

[dondonz]

Major renovation to the site, whilst maintaining all content and preserving existing URLs. See this fork live: https://priceless-haibt-31ae6b.netlify.app

Docusaurus is an easy-to-use docs generator, just add Markdown. It gives you all the nice features of React without requiring any knowledge of React nor JS. There's hot reloading, linter to check for dead links, awesome search support and sensible CSS defaults.

Docusaurus is actively maintained by Facebook https://docusaurus.io/. If you've used React, Relay, or React Native documentation lately, you've already seen Docusaurus in action.

Here's what's in this PR so far

• Next (master), current (v17), and previous (v16) documentation fully migrated. I went through and tidied up all the code formatting. Defaults to v17
• All blog posts migrated, with the same URLs
• About page migrated
• Fancy frontend renovations - colour scheme, better header and footer, edit buttons everywhere to encourage contributions
• Tutorial on separate tab

After initial ship

• Option to add more text on the landing page later
• Awaiting Algolia's reply on DocSearch, a daily automated crawler. Will add search after we receive our API key