refactor(deploy): let Cloudflare's Git integration own its deploys

The Pages project is connected to the repository directly, so an Actions
workflow running wrangler would deploy a second time on every push and
race it. Cloudflare builds itself; only the build command has to carry
the --config flag, which the dashboard supplies.

- remove .github/workflows/cloudflare-pages.yml and its secret setup
- document the required dashboard build command and build system v3
- note that manual wrangler deploys are rejected by Git-connected
  projects

.github/workflows/cloudflare-pages.yml

@@ -1,38 +0,0 @@
-name: Deploy to Cloudflare Pages
-
-on:
-  push:
-    branches: [main]
-  workflow_dispatch:
-
-permissions:
-  contents: read
-
-concurrency:
-  group: cloudflare-pages
-  cancel-in-progress: true
-
-jobs:
-  deploy:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v4
-
-      - uses: ruby/setup-ruby@v1
-        with:
-          ruby-version: '3.4'
-          bundler-cache: true
-
-      # Layers _config.cloudflare.yml on top, swapping the GitHub Pages subpath
-      # for the pages.dev root.
-      - name: Build site
-        run: bundle exec jekyll build --config _config.yml,_config.cloudflare.yml
-        env:
-          JEKYLL_ENV: production
-
-      # Project name and output directory come from wrangler.jsonc.
-      - uses: cloudflare/wrangler-action@v4
-        with:
-          apiToken: ${{ secrets.CLOUDFLARE_API_TOKEN }}
-          accountId: ${{ secrets.CLOUDFLARE_ACCOUNT_ID }}
-          command: pages deploy