CI: use new docs:build-ci command

For now, this only skips external link checking. But this can be easily
expanded in the future (lib/utility.js provides a isCIBuild() function
to check if this is a CI build).

.github/workflows/ci.yml

@@ -17,4 +17,4 @@ jobs:
         run: npm ci
       - name: Build with VitePress
         run: |
-          npm run docs:build
+          npm run docs:build-ci