[rustdoc] Page hash handling

Fixes https://github.com/rust-lang/rust/issues/70476

A good example to see the change is to use this URL: https://doc.rust-lang.org/nightly/std/string/struct.String.html#from_iter.v-3

After the change, it actually goes to the target element (and change the page hash to something more clear for the users).

r? @kinnison

cc @ollie27