Rewrite the "Custom Type Errors" guide for 0.12

[LiamGoodacre]

Based on the changes added here: https://github.com/purescript/purescript/pull/3298

Roughly:

• New Doc kind
• TypeConcat -> Beside
• TypeString -> Quote
• Symbol literal in message -> Text
• New vertical composition: Above

[i-am-tom]

[hdgarrood]

Now that the relevant types and kinds have their own module inside the compiler, I'd quite like to move this guide into the compiler so that it shows up on the docs page for Prim.TypeError on Pursuit - see #96.

[LiamGoodacre]

Sounds good :+1:

[JordanMartinez]

This folder explains the new custom type errors (since I don't think they are explained in Prim's docs yet). Also, one needs to understand TLP syntax first before their docs make more sense. https://github.com/JordanMartinez/purescript-jordans-reference/tree/latestRelease/21-Hello-World/04-Debugging/src/02-Custom-Type-Errors

[andyarvanitis]

I think even just a quick example (on Pursuit) would be helpful, at least for now. The guide is still out there and wrong, and it currently takes some digging to find the right way to do something like a simple warning. I'm happy to help, too.