docs: rewrite alternative browsers guide, drop pinned Lightpanda version

Install steps per engine and platform, one config to switch engines,
and latest-release download links instead of a hardcoded version.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: DavertMik

.github/workflows/lightpanda.yml

@@ -18,8 +18,6 @@ permissions:
 env:
   CI: true
   FORCE_COLOR: 1
-  LIGHTPANDA_VERSION: 1.0.0
-  LIGHTPANDA_SHA256: aa5a4b8ed53d1e38b3c73f5b2647d0a84a82e6744557f45f9a9c85858aa031c3
   LIGHTPANDA_DISABLE_TELEMETRY: true
 
 jobs:
@@ -44,18 +42,10 @@ jobs:
           php-version: 8.0
       - name: npm install
         run: npm i --force
-      - name: Cache Lightpanda binary
-        uses: actions/cache@v4
-        with:
-          path: ~/lightpanda-bin/lightpanda
-          key: lightpanda-${{ env.LIGHTPANDA_VERSION }}-x86_64-linux
       - name: Download Lightpanda
         run: |
           mkdir -p ~/lightpanda-bin
-          if [ ! -f ~/lightpanda-bin/lightpanda ]; then
-            curl -sfL --retry 3 -o ~/lightpanda-bin/lightpanda https://github.com/lightpanda-io/browser/releases/download/${LIGHTPANDA_VERSION}/lightpanda-x86_64-linux
-          fi
-          echo "${LIGHTPANDA_SHA256}  $HOME/lightpanda-bin/lightpanda" | sha256sum -c -
+          curl -sfL --retry 3 -o ~/lightpanda-bin/lightpanda https://github.com/lightpanda-io/browser/releases/latest/download/lightpanda-x86_64-linux
           chmod +x ~/lightpanda-bin/lightpanda
           ~/lightpanda-bin/lightpanda version
       - name: start test server