rustdoc: add doc_link_canonical feature

[lolbinarycat]

proposing this as a t-rustdoc experiment (do we have a process for those?), implementation was surprisingly simple so it's basically a complete implementation of my design (although ideally it would be integrated into cargo in the future using --crate-attr), will likely need FCP then RFC for stabilization, may also need perf testing to make sure added allocations have a negligible impact, not sure.

fixes #143139

r? @GuillaumeGomez

[rustbot]

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann

[lolbinarycat]

The one feature that isn't currently implemented is having the default value be the same as from html_root_url if you just specify doc(html_link_canonical) with no value.