Make all documentation fully available in hexdocs

[NelsonVides]

Currently, documentation for each rule is available only in github, and in hexdocs, the link to the rules document simply forwards to the github page, which contains links to all the rules with their details. We probably want to make it so that all documentation is fully available in hexdocs.

Spawns from #566 (comment)

[paulo-ferraz-oliveira]

Yeah, we've tried to have it there, in the past, but maybe we need some type of validation/automation, so we don't forget stuff when we add new rules. Nice issue for follow-up, though.