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.

Note we have to install redcarpet for YARD's Markdown support but
redcarpet doesn't compile on Alpine so we make an optional Bundler group
(see
https://bundler.io/guides/groups.html#optional-groups-and-bundlewith).

.github/workflows/docs.yml

@@ -0,0 +1,37 @@
+name: Build documentation
+
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - main
+
+jobs:
+  build:
+    runs-on: "ubuntu-latest"
+    env:
+      BUNDLE_WITH: docs
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: "3.2"
+          bundler-cache: true
+      - run: bundle exec yardoc
+      - 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 +3,8 @@ source "https://rubygems.org"
 gemspec
 
 gem "rake", "> 12.3.2"
+
+group :docs, optional: true do
+  gem "yard"
+  gem "redcarpet"
+end