Add CI job to ensure documentation can be built

Author: jplatte

.github/workflows/ci.yml

@@ -75,6 +75,16 @@ jobs:
       - uses: dtolnay/rust-toolchain@clippy
       - run: cargo clippy
 
+  docs:
+    name: Documentation
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: dtolnay/rust-toolchain@nightly
+      - run: cargo doc --features raw_value,unbounded_depth
+        env:
+          RUSTDOCFLAGS: --cfg docsrs
+
   fuzz:
     name: Fuzz
     runs-on: ubuntu-latest