Determine a standard way of generating LFE docs for release inclusion

[oubiwann]

Tasks:

• Generate docs in standard output format
  • before or after the release is generated?
  • probably before ...
  • how do you ensure that docs are generated with relx ... when calling rebar3 release?
• Place the generated docs in the release hierarchy ... or keep them in each project?
• Place the index file in the release hierarchy (in fact, this might be the only doc file that gets put at the olp level)
  Part of epic Standardize Release Documentation #28