feat: set Gateway 3 as root index and move dispatches to /dispatches/

- Update src/index.njk with Gateway 3 homelab services router and cluster telemetry
- Create src/dispatches.njk for the full-width technical dispatches feed and tag filters
- Update src/_data/site.json navigation hierarchy to reflect root Gateway and /dispatches/
- Update breadcrumbs and return links in post.njk and hero-tile.njk

src/_includes/partials/hero-tile.njk

@@ -6,7 +6,7 @@
   </p>
 
   <div class="filter-pills-row" aria-label="Topic Filters">
-    <a href="/" class="filter-tag-btn {% if not currentTag %}active{% endif %}">
+    <a href="/dispatches/" class="filter-tag-btn {% if not currentTag %}active{% endif %}">
       [All Dispatches ({{ collections.posts.length }})]
     </a>
     {% for tag in collections.tagList %}