Add a new docs provider

[starbelly]

We should add a docs provider ala mix hex.docs. Said provider should be capable of fetching docs for the users application, but also any dependencies the application declares.

In addition, the provider should offer convienence functions for opening the documentation in both online and offline form, as well as providing options around which format to fetch.

See https://hexdocs.pm/hex/Mix.Tasks.Hex.Docs.html for an example.