add fallback docs for crates

[poopsicles]

basically #151 (comment)

currently, if a crate has a docs value but no link, the custom docs are favoured over docs.rs/name

my editor auto-formatted, lemme know if you want me to turn that off

[djc]

Autoformatting is nice, but please put it in a separate commit? (Or PR if your prefer.)

[poopsicles]

there's some clippy stuff so ill make a followup PR for all that

[nicoburns]

I agree that auto-formatting should be a separate PR.

[poopsicles]

some crates have multiple sets of docs, the ones in bold are the ones i linked:

• rustup has the book and the dev guide
• clippy has the book and the lint docs
• cross has the docs/ subfolder and the wiki
• vizia has the docs and the book
• freya has the nightly docs, stable docs, and the book

[djc]

Thanks for digging this up, seems like a nice improvement!

[nicoburns]

Some of these crates that have links because they weren't published to crates.io yet now are published (xilem, parley, vizia). But this seems like a solid improvement anyway. We might want to look at a way to link to both guide and reference docs at some point. Although I'm wary of providing too many options.