Convert existing docs to Codox "Topics"

[oubiwann]

Wiki page:

• https://github.com/clojusc/friend-oauth2/wiki

Tasks:

• [x] copy original, committing as original author
• [x] update with change from other contributor
• [x] once doc is moved into codebase, split into different parts:
  • [x] installation
  • [x] usage
  • [x] testing
  • [x] todo

Part of feature #39