Only run docs CI on schedule or code change

Previously, docs CI was run on every code change, even when docs were
untouched. This tended to generate noise with many false positives.

Scheduled runs still catch when any existing links break, but at a
weekly frequency so that any false positives are less noisy.

Author: victorlin

.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