ci: use nightly for cargo docs

Author: mladedav

.github/workflows/CI.yml

@@ -31,7 +31,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
     - uses: actions/checkout@v5
-    - uses: dtolnay/rust-toolchain@stable
+    - uses: dtolnay/rust-toolchain@nightly
     - uses: Swatinem/rust-cache@v2
       with:
         save-if: ${{ github.ref == 'refs/heads/main' }}