Add Rust documentation check to GitHub Actions

[cycraig]

Description of change

Build Rust documentation in CI to catch broken links early. The new check is relatively fast at ~5 minutes, so should not slow down the overall CI, especially when run in parallel with much larger jobs.

Type of change

• Chore/Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

• Successful CI run: https://github.com/iotaledger/identity.rs/runs/7255904825?check_suite_focus=true
• Failed CI run (to show it catches broken intra-doc links, even from a README.md file): https://github.com/iotaledger/identity.rs/runs/7256033459?check_suite_focus=true

To test locally, change one of the documentation links in a Rust file to be incorrect.
Then run:

RUSTDOCFLAGS="-D warnings" cargo doc --all-features --no-deps --workspace

It should print an error similar to the one below and exit with code 1:

error: unresolved link to `crate::validator::PresentationValidator`
 --> identity_iota_client\src\lib.rs:6:1
  |
6 | #![doc = include_str!("./../README.md")]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `-D rustdoc::broken-intra-doc-links` implied by `-D warnings`
  = note: the link appears in this line:

          - [`PresentationValidator`](crate::validator::PresentationValidator)
                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = note: no item named `validator` in module `identity_iota_client`

error: could not document `identity_iota_client`

Change checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[PhilippGackstatter]

Can we build it the way docs.rs builds the documentation, so we don't run into the 0.6 doc problems again?
So specifically using nightly and --cfg docsrs. So:

RUSTDOCFLAGS='-D warnings --cfg docsrs' cargo +nightly doc --all-features --no-deps --workspace

[cycraig]

Good point, will do done.

Is the docsrs config really necessary in our code though?
For example, the double cfg features in identty_iota?

#[cfg(feature = "account")]
#[cfg_attr(docsrs, doc(cfg(feature = "account")))]
pub mod account_storage {

Edit: should we specify the features in Cargo.toml instead or something? Not sure what the best practice is.

E.g.

[package.metadata.docs.rs]
features = [ "account", "..." ]

Edit2: we currently have all-features though, so probably unnecessary.

[package.metadata.docs.rs]
all-features = true
rustdoc-args = ["--cfg", "docsrs"]

[PhilippGackstatter]

The double-cfg is necessary I think, since #[doc(cfg(...))] is unstable. It's how crypto.rs does it, too. I mainly copied their approach: https://github.com/iotaledger/crypto.rs/blob/94be7913f11018467edb64c3d126ab682c29edcf/src/signatures/mod.rs#L5.

Yes, I think all-features = true in the toml metadata should be fine.