Generate docs for hexpm

[gomoripeti]

Generate documentation that looks somewhat better than the default edoc style (look at elixir docs).
Figure out how to publish (eg. maybe both Erlang API from xprof_core and REST API/GUI guide from xprof_gui should be copied/duplicated under xprof app documentation).
What should be the source format of non-edoc pages/guides (eg markdown)

[izoslav]

Docs for version 1.3.0 (current master branch) are gerenrated and published to hexdocs.pm.

Unfortunately, it's impossible to publish docs for unpublished package versions, so we have to postpone work on 2.0 docs until full release.