docs: Split API docs into their own section

docs: Split API docs into their own section

Third time lucky. There are two changes:

- Add a new 'clients' section to the usage doc, allowing us to remove a
  lot of the API nitty gritty stuff from the users guide. This makes
  more sense as users don't really care what API method they're using -
  only what application).

- Change the API docs from the developers guide into a quick start
  section, allowing us to greatly expand the REST API docs to include
  information on pagination, authentication, etc.

Signed-off-by: Stephen Finucane <stephen@that.guru>