Add docs to CI

This is primarily to detect broken intra-doc links, which `cargo doc`
warns about.

Author: jturner314

.github/workflows/ci.yml

@@ -76,3 +76,17 @@ jobs:
           components: clippy
       - run: cargo clippy
 
+  docs:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        rust:
+          - stable
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: ${{ matrix.rust }}
+          override: true
+      - run: cargo doc