Warn against here-links in doc comments

[SabrinaJewson]

What it does

Warns when a user writes [here], [click here], [link] or [this link] in a doc comment. This lint would be in the “Style” category, as I think it is fairly universally agreed-upon that such links should be avoided (I have not seen arguments to the contrary, at least).

Advantage

Links of this form are widely considered bad for accessibility[1][2][3], among other reasons. It is recommended to change the link text to a short description of what is at the URL instead.

Drawbacks

Sometimes it is non-obvious how to rewrite a here-link into the preferred form, which could increase friction when writing docs.

Example

/// For more details on this function, click [here](crate).

Could be written as:

/// For more details on this function, see [the crate-level docs](crate).