chore: add Netlify doc previews

[davidbarsky]

This branch (tries) to setup tracing-style rustdoc previews in PRs; I'm not sure if it works yet. We'll see! I'll update this PR accordingly.

[carllerche]

Thanks. This will be super helpful to help review PRs. Sometimes, I just want to look at what the public API is and the docs can be an easier medium.

[netlify]

❌ Deploy Preview for tokio-rs failed.

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[hawkw]

some suggestions regarding the build configuration. once those are addressed, i think this will look good to me

[davidbarsky]

I'm getting the following error when running env RUSTDOCFLAGS="-D warnings --cfg docsrs --cfg tokio_unstable" RUSTFLAGS="--cfg tokio_unstable" cargo +nightly doc --no-deps. This occurs on at least rustc 1.63.0-nightly (4c5f6e627 2022-05-17) (from rustc 1.63.0-nightly (42e1761c7 2022-05-15)); I'll try other nighties before I run cargo-bisect-rustc.

error: unresolved link to `try_join`
  --> tokio/src/macros/join.rs:15:20
   |
15 | /// [`try_join!`]: macro@try_join
   |                    ^^^^^^^^^^^^^^ no item named `try_join` in scope
   |
   = note: `-D rustdoc::broken-intra-doc-links` implied by `-D warnings`
   = note: `macro_rules` named `try_join` exists in this crate, but it is not in scope at this link's location

error: could not document `tokio`

[davidbarsky]

Yup, looks related to rust-lang/rust#96676.

[hawkw]

yay!