Rollup merge of #140434 - a4lg:rustdoc-multi-footnote-refs, r=fmease,GuillaumeGomez

rustdoc: Allow multiple references to a single footnote

Multiple references to a single footnote is a part of GitHub Flavored Markdown syntax (although not explicitly documented as well as regular footnotes, it is implemented in GitHub's fork of CommonMark) and not prohibited by rustdoc.

cf. <https://github.com/github/cmark-gfm/blob/587a12bb54d95ac37241377e6ddc93ea0e45439b/test/extensions.txt#L762-L780>

However, using it makes multiple `sup` elements with the same `id` attribute, which is invalid per the HTML specification.

Still, not only this is a valid GitHub Flavored Markdown syntax, this is helpful on certain cases and actually tested (accidentally) in `tests/rustdoc/footnote-reference-in-footnote-def.rs`.

This commit keeps track of the number of references per footnote and gives unique ID to each reference.
It also emits *all* back links from a footnote to its references as "↩" (return symbol) plus a numeric list in superscript.

As a known limitation, it assumes that all references to a footnote are rendered (this is not always true if a dangling footnote has one or more references but considered a reasonable compromise).

Also note that, this commit is designed so that no HTML changes will occur unless multiple references to a single footnote is actually used.

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