For docs generation check, pin nightly to nightly-2025-09-27

[iamjpotts]

Resolves new ci failure introduced by nightly-2025-09-28:

 Documenting deranged v0.4.0
error[E0557]: feature has been removed
 --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/deranged-0.4.0/src/lib.rs:3:29
  |
3 | #![cfg_attr(docsrs, feature(doc_auto_cfg))]
  |                             ^^^^^^^^^^^^ feature has been removed
  |
  = note: removed in 1.58.0; see <https://github.com/rust-lang/rust/pull/138907> for more information
  = note: merged into `doc_cfg`

error: Compilation failed, aborting rustdoc

For more information about this error, try `rustc --explain E0557`.
error: could not document `deranged`
warning: build failed, waiting for other jobs to finish...
Error: Process completed with exit code 101.

Linked pr from error message:

rust-lang/rust#138907

Linked in comment about when to remove the new pin:

jhpratt/deranged#24

[iamjpotts]

CI seems to be stuck waiting for a non-existent job Documentation (nightly) to finish.

ci / Documentation (nightly-2025-09-27) and ci / Documentation (stable) both passed.

[iamjpotts]

@ctz posted a question at #382 (comment):

Is it even possible to pin the version of nightly that docs.rs uses?

But I understood the proposed fix to be about pinning it in our CI so the docs workflow can do its thing.

But isn't the docs workflow's purpose to demonstrate that the docs.rs build will work once the crate is published?

[iamjpotts]

@djc / @cpu - I assume we do not want this PR anymore now that #384 is merged?

Feel free to close it if so (I'll close it in a few days otherwise).