Move all documentation to this repo

Make this repo the only place for documentation for the entire DAOstack project.

How it will work:
- Documentation from all subprojects will be in this repo in a seperate folder (`/arc` for Arc, `/arc-js` for Arc.js)
- Documentation generated from source code will be generated locally and committed to this repo (not committed to subproject repo).
- A CI service will compile and deploy a single website to the `gh-pages` branch of this repo.

Benefits:
- Easy to document cross-cutting concerns (e.g. an end-to-end tutorial on how to use the whole DAOstack together).
- Easier linking between subprojects (with the ability to check for broken links between subprojects).
- Consistent look & feel using the same website.
- Removing unnecessary complexity from the subprojects repos i.e managing the website.
- Supports switching markdown compilers if we need to in the future (e.g. switch to [Jekyll](https://jekyllrb.com/docs/home/) or [read the docs](https://jekyllrb.com/docs/home/)) or changing the behavior of the website.