librustdoc: Use `pulldown-cmark-escape` for HTML escaping Implementation of `@notriddle` 's [suggestion](https://github.com/rust-lang/rust/pull/137274#issuecomment-2669001585). Somewhat related to #137274 , but the two PRs should be complementary. Local perf results look like a nice improvement! (so would love a perf run on the CI)