feat(cli/rustup-mode): allow rustup doc with both a flag and a topic

[rami3l]

This PR is my stab at the concern in #4069.

However, I think rustup should not magically detect and acknowledge lints, so I've made a more neutral solution that might apply to more topics:

Does rustup doc --rustdoc lints::broken_intra_doc_links look okay to you?

I mean, we can try with the following semantics with lints::broken_intra_doc_links:

1. Look for lints/broken_intra_doc_links/index.html.
2. Look for lints/broken_intra_doc_links.html.
3. Look for lints.html. If it exists, resolve to lints.html#broken_intra_doc_links.
4. Throw an error.

PS: I have manually verified that this algorithm, if implemented correctly, will work with rustc, rustdoc and clippy books.
#4069 (comment)

Concerns

• Should we add test cases for this PR, and how may we add them? Added --path tests to ensure that we can get the doc paths right.

[rami3l]

PS: The CI failures seem irrelevant to the changes being made in this PR.