Add weekly scheduled workflow to rebuild docs site

The docs site embeds content from external repositories (e.g. the
getting-started page is embedded from CAPI). This embedded content
only updates when Netlify rebuilds the site, which only happens on
push to main. If no PRs are merged for a while, the embedded content
becomes stale.

Add a weekly scheduled GitHub Actions workflow that triggers a Netlify
build via a build hook. This ensures the docs site always has the
latest content from embedded external sources.

Maintainers need to:
1. Create a build hook in Netlify (Site settings > Build & deploy >
   Build hooks)
2. Add the hook URL as a GitHub Actions secret named NETLIFY_BUILD_HOOK

Signed-off-by: Dong Ma <winterma.dong@gmail.com>

Author: larainema

.github/workflows/weekly-docs-build.yaml

@@ -0,0 +1,17 @@
+name: Weekly docs build
+
+on:
+  schedule:
+  # Run every Monday at 8:00 UTC.
+  - cron: "0 8 * * 1"
+  workflow_dispatch: {}
+
+# Remove all permissions from GITHUB_TOKEN except metadata.
+permissions: {}
+
+jobs:
+  trigger-netlify-build:
+    runs-on: ubuntu-slim
+    steps:
+    - name: Trigger Netlify build
+      run: curl -X POST -d '{}' "${{ secrets.NETLIFY_BUILD_HOOK }}"