ci: Check all crate documentation in addition to target-specific ndk

Only `ndk` docs are built for all `android` targets but the rest of the
documentation remains untested. Especially now that we're expanding to
provide more complete docs for ndk-build and cargo-apk should everything
be tested, to make sure that things like intra-doc links remain in
working order.

.github/workflows/rust.yml

@@ -73,6 +73,12 @@ jobs:
       run:
         cargo test -p cargo-apk --all-features
 
+    - name: Document all crates
+      env:
+        RUSTDOCFLAGS: -Dwarnings
+      run:
+        cargo doc --all --all-features
+
     - name: Install cargo-apk
       run:
         cargo install --path cargo-apk