Create playground / documentation site

[JohnnyMorganz]

README is currently very long, with a lot of words, and maybe a bit too verbose - it could grow even further in the future.
We should cut it down, so its easier to find what you're looking for. Nobody wants to be searching through text.

It may also be wise to start up a separate documentation page, so we can split it up and go into more detail about things. I want to document more of the specific style choices made, to make it easier to see what is happening. Only issue with a documentation site is remembering to keep it up to date.
mdBook looks simple and clean, or maybe we should just use GitHub Wiki?

I also want to start hosting a playground to make it easy to quickly test StyLua, similar to prettier's playground - it would also be useful for users to report issues by pasting a playground sample. Started some of this on the playground branch, and its somewhat working, just need to design it now. It also requires stylua to be built for wasm - need to remember to keep that up to date, and I don't know how easy it is to build wasm to compare e.g. current HEAD vs. a released version.