rustdoc & doc: no `shortcut` for `rel="icon"`
According to https://html.spec.whatwg.org/multipage/links.html#rel-icon:
> For historical reasons, the `icon` keyword may be preceded by the keyword "`shortcut`".
And to https://developer.mozilla.org/en-US/docs/Web/HTML/Link_types:
> **Warning:** The `shortcut` link type is often seen before `icon`, but this link type is non-conforming, ignored and **web authors must not use it anymore.**
While it was removed from the Rust logo case a while ago in commit 085679c ("Use theme-adaptive SVG favicon from other Rust sites"), it is still there for the custom logo case.
Also updated a few other instances.
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
c695de05f2