Rust logo on doc.rust-lang.org/std/* pages should link to www.rust-lang.org (or at least doc.rust-lang.org)

[eddyp]

When on pages on https://doc.rust-lang.org/std/* The Rust logo links to https://doc.rust-lang.org/**std**/index.html, but that should be the obvious way to return to doc.rust-lang.org or even rust-lang.org.

Under the Rust logo there is an element with the name of the current module, there could be a link to docs.rust-lang.org and doc.rust-lang.org/std above it, to not lose the link to the links to items "higher" in the hierarchy.