rustdoc: update to pulldown-cmark 0.11
r? rustdoc
This pull request updates rustdoc to the latest version of pulldown-cmark. Along with adding new markdown extensions (which this PR doesn't enable), the new pulldown-cmark version also fixes a large number of bugs. Because all text files successfully parse as markdown, these bugfixes change the output, which can break people's existing docs.
A crater run, https://github.com/rust-lang/rust/pull/121659, has already been run for this change.
The first commit upgrades and fixes rustdoc. The second commit adds a lint for the footnote and block quote parser changes, which break the largest numbers of docs in the Crater run. The strikethrough change was mitigated in pulldown-cmark itself.
Unblocks https://github.com/rust-lang/rust-clippy/pull/12876
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