fix: use nightly Rust for deploy-docs job to support doc_cfg feature

The deploy-docs job sets RUSTDOCFLAGS='--cfg docsrs' which activates
`#![cfg_attr(docsrs, feature(doc_cfg))]` in lib.rs. Since feature(doc_cfg)
is a nightly-only feature, the build fails with stable Rust 1.90:

  error[E0554]: `#![feature]` may not be used on the stable release channel

Switch the deploy-docs job to use nightly Rust, which is the standard
practice for documentation builds (docs.rs also uses nightly). The separate
'docs' CI check job remains on stable since it doesn't set the docsrs cfg.

Co-authored-by: Greg Nazario <greg@gnazar.io>

Author: cursoragent

.github/workflows/ci.yml

@@ -171,10 +171,8 @@ jobs:
     steps:
       - uses: actions/checkout@v6
 
-      - name: Install Rust
-        uses: dtolnay/rust-toolchain@master
-        with:
-          toolchain: "1.90"
+      - name: Install Rust nightly (required for doc_cfg feature)
+        uses: dtolnay/rust-toolchain@nightly
 
       - name: Cache cargo
         uses: Swatinem/rust-cache@v2