Pin the docs workflow to Swift 6.3

Without a version the setup action installs the latest toolchain.
Swift 6.4.0 shipped on 2026-09-15 and the swiftly build cached on the
runner cannot install it, so the docs job started failing.

.github/workflows/docs.yml

@@ -26,6 +26,7 @@ jobs:
     - name: Set up Swift
       uses: swift-actions/setup-swift@v3
       with:
+        swift-version: "6.3"
         skip-verify-signature: true
     - name: Generate Docs
       run: |