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/download/index.html

@@ -254,6 +254,7 @@ footer a{margin:0 8px}
     <p>
       <a href="../">Home</a>
       <a href="../guide/">Setup guide</a>
+      <a href="../features/">Features</a>
       <a href="../troubleshooting/">Troubleshooting</a>
       <a href="https://github.com/dzianisv/opencode-mobile">GitHub</a>
       <a href="https://dzianisv.github.io/opencode-mobile/fdroid/repo">F-Droid repo</a>