Docs site

## Describe the enhancement

Create a proper documentation site for Linaria, even just by putting current md docs into gitbook, for more legitimacy and community adoption.

## Motivation

I'm advocating for Linaria in my agency, and the lack of a proper docs site is making that harder. The assortment of markdown files thrown in the github repo makes the project feel much less mature and fleshed out than it is, especially for wider adoption throughout the dev team compared to no-brainers like Emotion.

I also think how a project is perceived is really important for community adoption, especially when Atlassian's [Compiled](https://compiledcssinjs.com/) is maturing and has a beautiful, robust documentation site, but an (imo) less elegant API compared to the much more established Linaria.

## Possible implementations

A proper site would be nice, but even just throwing the existing md files into a free [gitbook](https://www.gitbook.com/) project would be a drastic improvement. That's what I do for even my smallest side projects (eg: [Pollen](https://www.pollen.style)) since it's basically no more effort than just writing docs in github.