Misc doc changes

[kianmeng]

Besides other documentation changes, this commit ensures the generated
HTML doc for HexDocs.pm will become the source of truth for this Elixir
library and leverage on latest features of ExDoc.

[kianmeng]

@aforward Any updates on this?

[aforward]

Wow, I totally missed this PR sorry @kianmeng will take a look this week

[aforward]

I pulled in my updates for Elixir 1.13 and all the tests pass, going to merge shortly and then cut a new release

10:43 ~/sin/projects/doex (misc-doc-changes)$ mix test
===> Analyzing applications...
===> Compiling parse_trans
==> jason
Compiling 10 files (.ex)
Generated jason app
===> Analyzing applications...
===> Compiling certifi
===> Analyzing applications...
===> Compiling hackney
==> httpoison
Compiling 3 files (.ex)
Generated httpoison app
==> doex
Compiling 3 files (.ex)
Generated doex app
Excluding tags: [live: true]

......................................

Finished in 0.08 seconds (0.00s async, 0.08s sync)
13 doctests, 28 tests, 0 failures, 3 excluded

Randomized with seed 738242

[kianmeng]

🥳 🥳 🥳 🥳 🥳

There is also this PR, aforward/version_tasks#1