Upload Docs to hexdocs

[jalcine]

Leveraging the community's own documentation system gives us a few things:

• Better local documentation support with tools like Dash and Zeal,
• Automatically generated documentation on new releases.
• Easier discovery for users when looking for documentation at specific versions.

[jalcine]

I noticed that there is some documentation at hedocs now!

https://hexdocs.pm/maru/readme.html

Though not comprehensive, it's an excellent start.

Would it be cool if some of the pages from https://maru.readme.io/docs were folded into the module docs to have those pages fleshed out there? This gives us the benefit of adding better version links to documentation on hex.pm