Add a possibility to build crate's docs from a path or repository

[aleksuss]

There is one method to build crate's docs at this moment, as I know. A crate should be published at crates.io. It would be nice to have a possibility to provide path or repo for crate to build docs for it. It nice to have for checking docs build for errors before publishing at crates.io. Any thoughts ?

[jyn514]

Are you asking for a way to build docs from a local directory to make sure it works before publishing it? This was implemented in #460.