Add action to run doctest. (#3)

`cargo test --all-features` does not run doc-tests. For more information
see https://github.com/rust-lang/cargo/issues/6669.

.github/workflows/test.yml

@@ -26,6 +26,9 @@ jobs:
       # https://twitter.com/jonhoo/status/1571290371124260865
       - name: cargo test --locked
         run: cargo test --locked --all-features --all-targets
+      # https://github.com/rust-lang/cargo/issues/6669
+      - name: cargo test --doc
+        run: cargo test --locked --all-features --doc
   minimal:
     runs-on: ubuntu-latest
     name: ubuntu / stable / minimal-versions