👷 CI: Check doc build for individual packages separately

Currently we end up with a strange error of enabling all features in the
root workspace. I don't know what really causes it (could be a Cargo
bug) but I also don't see the need to enable all features in the root so
I don't want to bother with that.

.github/workflows/rust.yml

@@ -232,5 +232,11 @@ jobs:
         with:
           toolchain: stable
       - uses: Swatinem/rust-cache@v2
-      - name: Check documentation build
-        run: cargo --locked doc --all-features
+      - name: Check zvariant documentation build
+        run: cargo --locked doc --all-features -p zvariant
+      - name: Check zbus_names documentation build
+        run: cargo --locked doc --all-features -p zbus_names
+      - name: Check zbus documentation build
+        run: cargo --locked doc --all-features -p zbus
+      - name: Check zbus_xml documentation build
+        run: cargo --locked doc --all-features -p zbus_xml