Set up documentation website generation and deployment (#253)

Generating the website is done by Jekyll with a theme called `just-the-docs`. 
Deployment of the website is done via GitHub Pages.

See:
- https://github.com/jekyll/jekyll
- https://github.com/just-the-docs/just-the-docs

.github/workflows/build.yaml

@@ -2,8 +2,7 @@ name: Build and verify
 on:
   pull_request:
   push:
-    branches:
-      - 'master'
+    branches: [$default-branch]
 permissions:
   contents: read
 jobs:

.github/workflows/deploy-website.yaml

@@ -0,0 +1,45 @@
+name: Update `error-prone.picnic.tech` website content
+on:
+  pull_request:
+  push:
+    branches: [$default-branch]
+permissions:
+  contents: read
+  id-token: write
+  pages: write
+concurrency:
+  group: pages
+  cancel-in-progress: true
+jobs:
+  build:
+    runs-on: ubuntu-22.04
+    steps:
+      - name: Check out code
+        uses: actions/checkout@v3.0.2
+      - name: Configure Github Pages
+        uses: actions/configure-pages@v2.1.1
+      - name: Generate documentation
+        run: ./generate-docs.sh
+      - name: Build website with Jekyll
+        uses: actions/jekyll-build-pages@v1.0.5
+        with:
+          source: website/
+          destination: ./_site
+      - name: Validate HTML output
+        uses: anishathalye/proof-html@v1.4.1
+        with:
+          directory: ./_site
+          check_external_hash: false
+      - name: Upload website as artifact
+        uses: actions/upload-pages-artifact@v1.0.4
+  deploy:
+    if: github.ref == format('refs/heads/{0}', github.event.repository.default_branch)
+    needs: build
+    runs-on: ubuntu-22.04
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v1.2.1