ci: trigger docs preview on install script changes

The install script is symlinked into docs/public/ and served via GitHub
Pages, but changes to the root file didn't trigger the preview workflow
because the path filter only matched docs/**.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: betegon

.github/workflows/docs-preview.yml

@@ -4,6 +4,7 @@ on:
   pull_request:
     paths:
       - 'docs/**'
+      - 'install'
       - '.github/workflows/docs-preview.yml'
 
 permissions: