Setup a Docusaurus site

We recently launched a new [docs and site gen tool](https://docusaurus.io/) called Docusaurus. I've gotten some previews of the tool and its pretty awesome. I've been wanted to get a standalone site for IGListKit setup for a while. Working w/ some FB folks on logistics, but in the meantime there are a few things we need to sort out:

- We gen API docs with jazzy using our headers. I'd love to still use this b/c its so convenient and integrates w/ Xcode really well. I'd like to avoid having two distinct types of docs (API w/ jazzy and general w/ Docusaurus). If we can extend jazzy to export Docusaurus docs, that'd be amazing.
- Design!
- We've talked about making some new tutorials and even videos. Stay tuned!
- Migrate our guides + FAQs to Docusaurus

cc @manicakes