Add documentation for the website to this repo

[vdice]

The documentation for the platform installer (https://fermyon.dev) currently lives in a private repo (which we've been intending to make public).

However, instead of maintaining the documentation in a separate repo, let's move the docs here (under docs) instead.

TODO

• [x] Merge documentation https://github.com/fermyon/installer/pull/79
• [ ] Move any issues from the previous docs repo into this repo