rustdoc: Rewrite HtmlDocCk from Python to Rust and greatly improve XPath support

[fmease]

Supersedes PR #125780. See also https://rustc-dev-guide.rust-lang.org/rustdoc-internals/rustdoc-html-test-suite.html#limitations.
META: FIXME: Flesh out this issue. This can become a tracking issue.

Re. better XPath support, https://github.com/Paligo/xee or https://github.com/james-lg/skyscraper might be a good option? The former might be excellent but might be too heavyweight, the latter might be too incomplete. Needs investigation.

[lolbinarycat]

Yeah, we're gonna need a discussion of what libraries to use for html+xpath because I don't see any obvious well-known choice.

[fmease]

We could also step back and think about alternatives to XPath like CSS selectors. Just throwing it out there. Would need a cost / benefit analysis, of course.

[lolbinarycat]

CSS selectors would let us use scraper, which would be nice.

[fmease]

The idea to use CSS selectors instead has really grown on me, we should probably pitch it on Zulip?

[fmease]

#t-rustdoc > CSS selectors instead of XPath for rustdoc-html tests? @ 💬

[GuillaumeGomez]

kuchiki is used by docs.rs and is quite nice.