Add lint for broken doc links

[maxcnunes]

fixes #2179

changelog: [doc_broken_link]: Add pedantic lint to catch broken doc links that won't produce a link tag by rustdoc.

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[maxcnunes]

Just noticed there are tests failing due to a false positive like this:

/// Referencing an slice [T]

This will be considered a broken link although actually it isn't. I guess in order to fix it I won't be able to check fake value from pulldown_cmark::Parser::new_with_broken_link_callback anymore, since it doesn't provide the raw text to check why it was considered a broken link. I will try to work on a different solution, appreciate if there are any suggestions to achieve it.

[maxcnunes]

I will try to use similar approach used on https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/tabs_in_doc_comments.rs

[bors]

☔ The latest upstream changes (presumably 8298da7) made this pull request unmergeable. Please resolve the merge conflicts.