Generate docs for hex.pm

[oubiwann]

Tasks:

• [ ] As with the lanes project, let's use mdbook for this (update Makefile, etc.)
• [ ] Move documentation content out of README
• [ ] Keep eye candy
• [ ] Use GH pages (main branch, /docs dir)
• [ ] During publish to hex.pm, create temp ./doc/index.html with a redirect to GH pages