Docs: Include more files, add ex_doc as a direct dependency

[adamu]

I was reading the docs for this project on Hexdocs, but I missed out on some information, as the useful info in the README is only in the repo, not the compiled docs.

This PR adds the README.md and RELEASE.md files to the compiled docs, and makes the README, rather than the Earmark module, the first page when visiting the docs.

Additionally, as ExDoc no longer depends on this project, it's possible to use ExDoc directly here, which I did and removed the ex_doc escript.

[crertel]

I'm not sure why ex_doc wasn't in as a dep, but I think it might be because ex_doc uses earmark internally, no?

[adamu]

I'm not sure why ex_doc wasn't in as a dep, but I think it might be because ex_doc uses earmark internally, no?

ExDoc used to use the Earmark.Parser module from this project - but that was forked to its own package which ExDoc now uses, so it's possible to use ExDoc directly here (but not in :earmark_parser).

[RobertDober]

@crertel you are right that was the reason and @adamu corrected it \o/