ci: add docsrs cfg option

Author: mladedav

.github/workflows/CI.yml

@@ -42,7 +42,7 @@ jobs:
     - name: cargo doc -p axum-extra
       run: cargo doc --package axum-extra --all-features --no-deps
     env:
-      RUSTDOCFLAGS: "-D rustdoc::all -A rustdoc::private-doc-tests"
+      RUSTDOCFLAGS: "-D rustdoc::all -A rustdoc::private-doc-tests --cfg docsrs"
 
   cargo-hack:
     runs-on: ubuntu-24.04