Platform switch should attempt to preserve current module

[nagisa]

Being in some module and then attempting to switch to a different platform (try unknown-linux-gnu) throws you back to the root of documentation.

It would be nice if an attempt was made to not forget the module user is in when switching the platform.

A naive implementation strategy could be something along the lines of: Given an URI like https://docs.rs/{crate}/{version}/{platform}/{...path} try to load https://docs.rs/{crate}/{version}/{new-platform}/{...path} and if that fails, load the crate root documentation.