feature request: build docs for dependencies only

[letheed]

Idea: add a new flag to cargo-doc doing the opposite of --no-deps (e.g. --deps-only).
It would be useful to be able to do

cargo doc --deps-only --open

to generate the documentation for the dependencies of the crate, even when the crate itself doesn't compile.

[KalitaAlexey]

I like that idea because sometimes my code does not compile because of errors to fix those I need a documentation for some external crate.

[KalitaAlexey]

I'll take it.

[kellda]

This is a duplicate of #2644 so maybe it can be closed ?

[epage]

This issue is specifically for documentation which has special dependency handling flags already like --no-deps. #2644 is broader build feature that is likely different enough in the needs there that we shouldn't combine the two.

For me, the question is how much is this

• Wanting to have documentation targeted at me without taking the time to build local dependencies
• Wanting have easy access to browsing what documentation did build when there is a failure (--keep-going can help for other workspace members)

if its the latter, I wonder if there is some level of still reporting or --opening we should do on error.