rustdoc: Allow paths to (partially) be ignored in links

It's very common for me to refer to a particular method, function, struct, etc without wanting to mention the full path. Right now the easiest way to do this is as following:

/// See also [`my_long_function_name`](some::path::to::my_long_function_name).

This is really common in my experience (most often with Self as the path), and has three main problems:

1. It's annoying to repeat yourself.
2. It's error prone to repeat yourself, you could make a typo/refactor in one or the other causing them to go out of sync.
3. It's annoying to repeat yourself.

In this issue I would like to propose a simple shorthand for the above:

/// See also [some::path::to::`my_long_function_name`].

In essence, I propose that as a preprocessing step we transform Markdown links of the form

[ <PRIVATE_PATH> :: ` <PUBLIC_PATH> ` ]

into

[ ` <PUBLIC_PATH> ` ] ( <PRIVATE_PATH> :: <PUBLIC_PATH> )

This makes writing links a lot more convenient in general, and less error prone. Two more examples, also highlighting the possibility to partially ignore a path:

[foo::`bar`]        ->    [`bar`](foo::bar)
[foo::`bar::baz`]   ->    [`bar::baz`](foo::bar::baz)

Start in rustdoc's intra-doc link handling and trace how Markdown links are parsed and resolved. Compare the proposed [foo::bar] and [foo::bar::baz] forms with the existing explicit-label syntax, then add coverage showing that the rendered label is shortened while the full path remains the link target.