Switch docs from MkDocs to custom SSG

[LPGhatguy]

Right now we embed the output of MkDocs, which doesn't fit in with the rest of Rojo's branding and is a little bit jarring.

Instead, we should consume all the MarkDown documentation and embed it into our nifty custom theme like Roblox TS does with their docs.

[unix-system]

roblox-ts now actually uses Docosaurus with custom theming; I think this approach would be better than spending the effort making our own SSG template, even if it would look niftier.

I'm happy to open a PR that achieves this; my main reasons for liking Docosaurus are as follows:

• Looks really nice out of the box
• We can version the documentation so we don't have to keep the 0.4.x and 0.5.x versions under categories in the main docs, which can lead to user confusion
• roblox-ts uses it, so reading both documentation sets simultaneously would be less jarring
• We can introduce custom styling that (roughly) matches the main site. Or, better yet, we can embed the main site into the Docosaurus template itself, so we're unified under one navbar.
• Comes with a sweet blog plugin built in, so finally we can have a blog! 😛

Thoughts?

[LPGhatguy]

Done, thank you for this, @unix-system!