ci: temporarily exercise fork docs build on PR 2691

Author: fengmk2

.github/workflows/build-docs-fork-preview.yml

@@ -43,9 +43,13 @@ jobs:
   build:
     name: Build fork docs
     needs: test
+    # TEMP: Exercise this job on PR #2691. Remove the exception before merge.
     if: >-
       github.repository == 'voidzero-dev/vite-plus' &&
-      github.event.pull_request.head.repo.full_name != github.repository
+      (
+        github.event.pull_request.head.repo.full_name != github.repository ||
+        github.event.pull_request.number == 2691
+      )
     runs-on: ubuntu-latest
     timeout-minutes: 15
     permissions: