Add a GitHub action for building docs. (#89)

This helps ensure we don't break that doc build.

.github/workflows/test.yml

@@ -26,3 +26,9 @@ jobs:
         with:
           command: test
           args: --no-default-features --features "json tls ${{ matrix.feature }}"
+      - name: Docs
+        uses: actions-rs/cargo@v1
+        with:
+          command: doc
+          # Keep in sync with Cargo.toml's [package.metadata.docs.rs]
+          args: --no-default-features --features "tls json charset cookies socks-proxy"

Cargo.toml

@@ -11,6 +11,7 @@ categories = ["web-programming::http-client"]
 edition = "2018"
 
 [package.metadata.docs.rs]
+# Keep in sync with .github/workflows/test.yml
 features = [ "tls", "json", "charset", "cookies", "socks-proxy" ]
 
 [features]