docs: dedupe homepage subtitle and description into _config.yml

Remove redundant front matter from index.pug; theme falls back to site config for banner and meta tags.

Co-authored-by: Cursor <[email protected]>

docs/source/index.pug

@@ -1,6 +1,4 @@
 layout: index
-description: A simple, expressive, extensible Liquid template engine for JavaScript — Shopify, Jekyll and GitHub Pages compatible, for Node.js, browsers, and the CLI, with TypeScript support.
-subtitle: A simple, expressive, extensible Liquid template engine for JavaScript
 ---
 ul#intro-feature-list
   li.intro-feature-wrap

docs/themes/navy/layout/index.swig

@@ -1,6 +1,6 @@
 <header id="banner" class="wrapper">
   <div class="inner inner-content">
-    <h2 id="banner-title">{{ page.subtitle }}</h2>
+    <h2 id="banner-title">{{ page.subtitle | default(config.subtitle) }}</h2>
     <div id="banner-share">{{ partial('partial/share') }}</div>
     <div id="banner-start">
       <code id="banner-start-command">npm install liquidjs</code><a id="banner-start-link" href="./tutorials/setup.html"><i class="icon-arrow-right"></i></a>