doc_suspicious_footnotes: lint text that looks like a footnote (#14708)

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

This is an alternative to https://github.com/rust-lang/rust/pull/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](https://gist.github.com/notriddle/59072476c9c1fd569fee421270dad665)
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](https://gist.github.com/notriddle/74eb8c9e1939b9f5c5549bf1d4fa238a)
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

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