Generate a docset from the cargo build cache

[Robzz]

Is your feature request related to a problem? Please describe.
There currently is no way of generating the docsets for multiple crates at once unless those crates are part of the same workspace.

Describe the solution you'd like
Generate docsets for many crates at once by leveraging the cargo build cache. cargo-docset would have to run over the whole build cache. It should also be possible to generate a single docset containing the documentation for the whole build cache. Gotta play with the build cache a bit before settling on how to do this, since I've never really used it.

Additional context
What should the feature look like from a UX perspective ? We could expand the generate command to make it able to achieve this, but it needs to behave so differently in this situation that I'm thinking it might be better as a different command, say generate-cache. The user could then use this feature with the following invocation: cargo docset generate-cache <options>.
The generate command would remain the default, and the simple cargo docset invocation would behave the same as it currently does.

I need to at the very least sleep on this though :)