Add baseurl-aware Pagefind bundlePath tag for GitHub Pages project sites

[Copilot]

Pagefind UI was resolving bundles from /pagefind/ by default, which breaks Jekyll sites deployed under url + baseurl (common on GitHub Pages project sites). This change makes bundle path resolution explicit and baseurl-aware while keeping indexing output behavior unchanged.

• Plugin runtime path helper

  • Added Jekyll::Pagefind.bundle_path(config) to compute a normalized path from:
    • site.baseurl
    • jekyll_pagefind.output_subdir (default: pagefind)
  • Ensures stable output format: /<baseurl?>/<output_subdir>/ with normalized slashes.

• Liquid integration

  • Added {% pagefind_bundle_path %} tag (BundlePathTag) to expose the computed bundle path directly in templates.

• Documentation updates

  • Updated Default UI example to pass bundlePath explicitly.
  • Added guidance that bundlePath must be baseurl-aware when baseurl is set.
  • Added note for Component UI JS initialization to use the same path.

<script src="{{ '/pagefind/pagefind-ui.js' | relative_url }}"></script>
<link rel="stylesheet" href="{{ '/pagefind/pagefind-ui.css' | relative_url }}" />

<div id="search"></div>
<script>
  window.addEventListener('DOMContentLoaded', () => {
    new PagefindUI({
      element: "#search",
      showSubResults: true,
      bundlePath: "{% pagefind_bundle_path %}"
    });
  });
</script>