doc_suspicious_footnotes: lint text that looks like a footnote

[notriddle]

changelog: [doc_suspicious_footnotes]: lint for text that looks like a footnote reference but has no definition

This is an alternative to rust-lang/rust#137803, meant to address the concerns about false positives.

This lint only fires when the apparent footnote reference has a name that's made from pure ASCII digits. This choice is justified by running lintcheck on the top 200 crates, plus the clippy default set:

1. I ran lintcheck with a modded version of this lint that didn't check for digits only. It produced a false positive warning on a line in mdbook that had a regex, and no true positives at all.
2. I also ran lintcheck with a custom lint that fired on any valid footnote reference with a non-ascii-digit name. cargo uses one in its job_queue module, and that's all it found.

cc @GuillaumeGomez

[rustbot]

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[GuillaumeGomez]

Looks good to me, thanks!

r? samueltardieu

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[blyxyas]

Great initial patch (and a really good lint idea 👍)! I left some suggestions and some questions.

[blyxyas]

Could this also check for #![doc], //!, #[doc] and the include!s macros?

[notriddle]

@blyxyas Okay, I've implemented variants on all your suggestions.