CI: add cargo doc run using stable features

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

.github/workflows/build.yml

@@ -70,6 +70,13 @@ jobs:
       with:
         command: doc
 
+    - name: "Run cargo doc for stable"
+      uses: actions-rs/cargo@v1
+      with:
+        command: doc
+        args: --no-default-features --features external_asm,instructions
+      if: runner.os != 'Windows'
+
     - name: "Run cargo build for stable without instructions"
       uses: actions-rs/cargo@v1
       with: