add an online mode for `rustup doc`

Problem you are trying to solve

rustup doc is very useful, but has a lot of edge cases. if you don't have rust-docs installed or the documentation is not packaged through static.rust-lang.org, you won't be able to read it. this requires people to download more, but more importantly, it greatly limits the number of places rustup can link (a few examples off the top of my head that are necessarily missing: crates.io, docs.rs, play.rust-lang.org, the security policy).

Solution you'd like

it would be great if rustup had a mode for opening online docs as well. my suggestion is something like this:

for existing docs, fallback to the online version if they're not installed locally (rustup already knows the channel names from the toolchain names so this should be fairly simple). if desired, rustup could have an --offline flag to disable this behavior.

for online-only docs, open the page directly (and give a hard error if --offline is passed)

for a list of links i think should be added, https://rust.tf/ is a good start - some of those are biased towards contributors, not users, but a lot of them are useful to both.

Notes

No response

Start by tracing the rustup doc command and how it handles missing local documentation. Define the online documentation links and the behavior for existing and online-only docs, including the proposed --offline option; done means these cases are specified and covered by appropriate tests.