feat: add djot parsing (DRAFT)

[ryanabx]

NOTE: I'm looking for feedback on this PR! This is my first time using Ruby :)

This PR aims to add support for djot HTML rendering for READMEs and whatnot.

There are many different implementations of the parser, but none of them are in Ruby unfortunately.

The best one I think we should use is djot.js, which is the most "official" parser for djot.

What I probably need to add still:

• [ ] tests for djot
• [ ] guarantee that node/npm is installed for users using github markup?

I will need some advice on how this is done in this repo. Any help is much appreciated!