Add logging in rustc itself that links to the dev-guide

It would really cool if, among the myriad other logging in rustc, we could also have 'dev-guide logging'. The idea is that you could have RUSTC_LOG=[rustc_dev_guide]=info, which shows logging from all crates that has a rustc_dev_guide span. I'm imagining something like this:

$ RUSTC_LOG=[rustc_dev_guide]=info rustc x.rs
INFO rustc_query_system[rustc_dev_guide] See https://rustc-dev-guide.rust-lang.org/query.html for more information about queries
INFO rustc_metadata[rustc_dev_guide] See https://rustc-dev-guide.rust-lang.org/serialization.html for more information about serialization in rustc

cc @hawkw - do you know if it's currently possible to have a span without a target? I'd hate to have to specify all 50 rustc_* crates by hand.
cc @igaray - I think you said you had a partial prototype for this?

Start by reviewing the proposed RUSTC_LOG command and the linked tracing-subscriber EnvFilter documentation. Determine how rustc crates could expose dev-guide spans and which guide pages should be linked; done means the requested filter produces useful logs across relevant crates.