Automatically update docs site

Rather than manually updating the docs at https://mudge.name/re2,
configure GitHub Actions to do it automatically on every push to the
main branch.

.github/workflows/docs.yml

@@ -0,0 +1,35 @@
+name: Build documentation
+
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - main
+
+jobs:
+  build:
+    runs-on: "ubuntu-latest"
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: "3.2"
+          bundler-cache: true
+      - run: bundle exec yard
+      - uses: actions/upload-pages-artifact@v2
+        with:
+          path: "doc/"
+
+  deploy:
+    needs: build
+    permissions:
+      pages: write
+      id-token: write
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    runs-on: ubuntu-latest
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v2

Gemfile

@@ -3,3 +3,4 @@ source "https://rubygems.org"
 gemspec
 
 gem "rake", "> 12.3.2"
+gem "yard"