seo(site): add /troubleshooting/ page (connection-error queries) + internal links

Authentic troubleshooting content mapping the app's six connection-failure
diagnostics (malformed-url, no-internet, tls, health-failed, timeout,
server-unreachable) to concrete fixes. Targets real problem queries
('opencode mobile can't connect', 'opencode tailscale setup', 'network request
failed'). FAQPage + BreadcrumbList JSON-LD for rich results. Linked from landing,
guide, and download footers; added to sitemap.

Co-Authored-By: Claude Opus 4.8 <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="../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>
       <a href="../privacy/">Privacy policy</a>