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.

List of changes:

• update formatters.exs and .gitignore from the latest Elixir mix new template
• add missing LICENSE.md file
• add relevant badges for quick overview and navigation to external sites
• fix markdown usage
• use latest ex_doc to leverage latest features

[kianmeng]

@mmcc @mupkoo Any updates on this?

[mmcc]

Hey @kianmeng thank you so much for the PR! Sorry for the delay. I need to catch up on some of these changes and what's new in mix.exs world these days, but we should be able to get this pulled in pretty quick.

Can you say a little more about what these changes actually do on the ExDoc side of things?

[kianmeng]

@mmcc Sure. I've updated the description of the PR to list all changes. Please check. Thanks.