Implement previews for GitHub pull requests

When a contributor submits a PR, we always perform a build. This takes
it a step further and uploads that a custom surge.sh domain. It adds a
sticky comment to link to that preview while also generating some diffs.
This means reviews easier.

In the implementation an additional preview step is added. This first
builds the base (target of the PR) as the current. Then it downloads the
generated preview that was added as an artifact in the original build
step. Creating a reasonably sized diff was tricky, because there's a
long Javascript line that includes the mtime, making it indeterministic.
That line isn't relevant anyway, so it's removed. The diff command also
ignores the search index.

All of that is placed in the preview, making it readable. A sticky
comment is added with a summary, making it easy to use. The sticky
comment is updated for every push, rather than added a comment for every
push. This keeps the PR conversation usable.

.github/workflows/test.yml

@@ -15,3 +15,87 @@ jobs:
         run: |
           npm install
           npx honkit build
+
+      - name: Upload artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: github-pages-pr-${{ github.event.pull_request.number }}
+          path: _book/**
+
+  preview:
+    runs-on: ubuntu-latest
+    needs: build
+    steps:
+      - name: Set preview domain
+        run: echo "PREVIEW_DOMAIN=$(echo ${{ github.repository }} | tr / - )-${{ github.job }}-pr-${{ github.event.pull_request.number }}.surge.sh" >> $GITHUB_ENV
+
+      - name: Install surge
+        run: npm install surge
+
+      - name: Install diffstat and pygments
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y diffstat python3-pygments
+
+      # TODO: can this download from the existing pages or a cache?
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          repository: ${{ github.event.pull_request.base.repo.full_name }}
+          ref: ${{ github.event.pull_request.base.ref }}
+          persist-credentials: false
+
+      - name: Build current pages
+        run: |
+          npm install
+          npx honkit build
+          mv _book current
+
+      - name: Download preview pages
+        uses: actions/download-artifact@v4
+        with:
+          # TODO: store output in upload step?
+          name: github-pages-pr-${{ github.event.pull_request.number }}
+          path: preview
+
+      - name: Remove indeterminism
+        run: |
+          find current/ preview/ -type f -exec sed -i '/gitbook.page.hasChanged/d' {} +
+
+      - name: Create diff to current
+        run: |
+          diff -Nrwu --exclude search_index.json current/ preview/ | cat > preview/diff.patch
+          if [[ -s preview/diff.patch ]] ; then
+            pygmentize -o preview/diff.html -l diff -f html -O full preview/diff.patch
+            diffstat -l -p2 preview/diff.patch > diff.txt
+          fi
+
+      - name: Deploy to surge.sh
+        run: npx surge ./preview $PREVIEW_DOMAIN --token ${{ secrets.SURGE_TOKEN }}
+
+      - name: Generate summary
+        run: |
+          echo "The PR preview for ${{ github.event.pull_request.head.sha }} is available at [${{ env.PREVIEW_DOMAIN }}](https://${{ env.PREVIEW_DOMAIN }})" > pr.md
+          echo "" >> pr.md
+
+          if [[ -f preview/diff.txt ]] ; then
+            echo "The following output files are affected by this PR:" >> pr.md
+            sed -E "s#(.*)#- [\1](https://${{ env.PREVIEW_DOMAIN }}/\1)#" preview/diff.txt >> pr.md
+          else
+            echo "No diff compared to the current website" >> pr.md
+          fi
+
+          if [[ -s preview/diff.patch ]] ; then
+            echo "" >> pr.md
+            echo "[show diff](https://${{ env.PREVIEW_DOMAIN }}/diff.patch)" >> pr.md
+          fi
+
+          if [[ -f preview/diff.html ]] ; then
+            echo "" >> pr.md
+            echo "[show diff as HTML](https://${{ env.PREVIEW_DOMAIN }}/diff.html)" >> pr.md
+          fi
+
+      - name: Comment on PR
+        uses: marocchino/sticky-pull-request-comment@v2
+        with:
+          path: pr.md