deploy: use GitHub Actions Pages workflow (#934)

This PR adds a `deploy.yml` to match [what's in our template repository](https://github.com/just-the-docs/just-the-docs-tests). I think it would be a great idea for us to match parity, especially:

- so that it's easier for us to help users & debug
- so that if we move to Jekyll 4 (likely soon), the transition is seamless
- so that we can explore adding custom items to the build step

Before merging this, we'll also need to change the source of the Pages deploy to actions (from the current branch deploy).

.github/workflows/deploy.yml

+# This workflow uses actions that are not certified by GitHub.
+# They are provided by a third-party and are governed by
+# separate terms of service, privacy policy, and support
+# documentation.
+
+# Sample workflow for building and deploying a Jekyll site to GitHub Pages
+name: Deploy Jekyll site to Pages
+
+on:
+  push:
+    branches: ["main"]
+
+  # Allows you to run this workflow manually from the Actions tab
+  workflow_dispatch:
+
+# Sets permissions of the GITHUB_TOKEN to allow deployment to GitHub Pages
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+# Allow one concurrent deployment
+concurrency:
+  group: "pages"
+  cancel-in-progress: true
+
+jobs:
+  # Build job
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+      - name: Setup Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.1' # Not needed with a .ruby-version file
+          bundler-cache: true # runs 'bundle install' and caches installed gems automatically
+          cache-version: 0 # Increment this number if you need to re-download cached gems
+      - name: Setup Pages
+        id: pages
+        uses: actions/configure-pages@v2
+      - name: Build with Jekyll
+        # Outputs to the './_site' directory by default
+        run: bundle exec jekyll build --baseurl "${{ steps.pages.outputs.base_path }}"
+        env:
+          JEKYLL_ENV: production
+      - name: Upload artifact
+        # Automatically uploads an artifact from the './_site' directory by default
+        uses: actions/upload-pages-artifact@v1
+
+  # Deployment job
+  deploy:
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    runs-on: ubuntu-latest
+    needs: build
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v1