have different docs for nightly/beta/stable channels

Currently clippy suggestion urls point to mostly the same page regardless of rustc release channel

stable:

    = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#unreadable_literal

beta:

    = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/master/index.html#unnecessary_mut_passed

nightly:

    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unreadable_literal

If a lint gets removed/downgraded in the current master branch and the docs get updates, the clippy version used on stable might not reflect this but still points to the master docs.
I think we need separate docs for nightly/stable/beta branches of the compiler.
Perhaps we can use separate branches for each release (like 1.30) or channel?