doc_link_code: add check for links with code spans that render weird

[notriddle]

This is the lint described at rust-lang/rust#136308 (comment) that recommends using HTML to nest links inside code.

changelog: [doc_link_code]: warn when a link with code and a code span are back-to-back

[rustbot]

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[notriddle]

cc @GuillaumeGomez

[GuillaumeGomez]

Seems surprising to make it a clippy lint. Why not a rustdoc lint instead?

[notriddle]

Implementing it in Clippy gives us a chance to shake out any unforeseen corner cases before uplifting it in Rustdoc.