feat: add Vercel-native deploy previews (#495)

- preview-build.sh: minimal Hugo build using current PR's integration
  files cloned into haystack-home — no tutorials/cookbook/advent steps
- vercel.json: wire up build command and output directory
- remove GHA preview.yml: Vercel handles PR comments and check status

Connect deepset-ai/haystack-integrations as a new Vercel project to
activate previews.

Author: kacperlukawski

.github/workflows/preview.yml

@@ -0,0 +1,18 @@
+#!/bin/bash
+set -euo pipefail
+
+HUGO_VERSION="0.155.2"
+wget -q "https://github.com/gohugoio/hugo/releases/download/v${HUGO_VERSION}/hugo_extended_${HUGO_VERSION}_linux-amd64.tar.gz"
+tar -xzf "hugo_extended_${HUGO_VERSION}_linux-amd64.tar.gz" hugo
+export PATH="$PWD:$PATH"
+
+git clone --depth=1 https://github.com/deepset-ai/haystack-home.git _site
+cd _site
+
+cp ../integrations/*.md content/integrations/
+cp -R ../logos/* static/logos/ 2>/dev/null || true
+
+npm install
+
+PREVIEW_URL="${VERCEL_URL:-localhost}"
+hugo -b "https://${PREVIEW_URL}"

preview-build.sh

@@ -0,0 +1,4 @@
+{
+  "buildCommand": "./preview-build.sh",
+  "outputDirectory": "_site/public"
+}