improvement(ci): run build-docs CI check for external pull requests (#…

…955)

* build: run build-docs for pull requests in addition to branch pushes

* docs(ci): add a helpful comment about why we have a condition in our build-docs job

.github/workflows/build-docs.yaml

@@ -7,9 +7,15 @@ on:
       - "!main"
     tags-ignore:
       - "*"
+  pull_request:
 
 jobs:
   build-docs:
+    # For internal PRs, this will run on `push` events;
+    #   for external/forked PRs, it will run on `pull_request` events.
+    #   Ideally we'd have checked this condition when defining the triggers,
+    #   but that's not supported by GitHub actions.
+    if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.fork
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2