Add a version dropdown to the docs page

[mathstuf]

Since Rust doesn't track "available since" in its documentation, it'd be nice to be able to switch to the documentation for the version I'm targeting easily. An example can be seen in the CMake docs where the dropdown lets you easily switch between documentation versions.

[pietroalbini]

Duplicate of #44687.

[mathstuf]

I don't know that it's a duplicate. Is that going to have an easy way to switch between versions other than URL editing?

[pietroalbini]

Yep, it's mentioned in the related RFC :)

[mathstuf]

Thanks for the pointer :) .