Only run docs CI on schedule or code change

.github/workflows/ci.yaml

@@ -345,13 +345,6 @@ jobs:
 
       - uses: ./src/.github/actions/run-integration-tests
 
-  doc:
-    uses: nextstrain/.github/.github/workflows/docs-ci.yaml@master
-    with:
-      docs-directory: doc/
-      pip-install-target: .[dev]
-      make-target: dirhtml
-
   release:
     # Restricted to version tags by the "on: push: tags: …" config at the top.
     if: |2

.github/workflows/docs.yaml

@@ -0,0 +1,29 @@
+name: Docs
+
+on:
+  push:
+    branches:
+      - master
+    paths:
+      - doc/**
+      - .github/workflows/docs.yaml
+
+  pull_request:
+    paths:
+      - doc/**
+      - .github/workflows/docs.yaml
+
+  workflow_dispatch:
+
+  # Routinely check that we continue to work in the face of external changes.
+  schedule:
+    # Every Monday at 17:42 UTC / 9:42 Seattle (winter) / 10:42 Seattle (summer)
+    - cron: "42 17 * * 1"
+
+jobs:
+  ci:
+    uses: nextstrain/.github/.github/workflows/docs-ci.yaml@master
+    with:
+      docs-directory: doc/
+      pip-install-target: .[dev]
+      make-target: dirhtml