seo(site): add /features/ page (capabilities, use cases, comparison)

Targets "opencode mobile features", "AI coding android", and
"opencode vs ChatGPT for coding" queries not covered by existing pages.
Includes SoftwareApplication + BreadcrumbList structured data.
Adds /features/ to sitemap and internal footer nav on all 5 pages.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs-site/sitemap.xml

@@ -15,6 +15,11 @@
     <changefreq>monthly</changefreq>
     <priority>0.8</priority>
   </url>
+  <url>
+    <loc>https://dzianisv.github.io/opencode-mobile/features/</loc>
+    <changefreq>monthly</changefreq>
+    <priority>0.8</priority>
+  </url>
   <url>
     <loc>https://dzianisv.github.io/opencode-mobile/troubleshooting/</loc>
     <changefreq>monthly</changefreq>