wiki: add alternatives section with SearXNG, FreshRSS, and Gitea

Add three new articles to 03-opensource/alternatives/:
- SearXNG: private metasearch, Open WebUI integration
- FreshRSS: self-hosted RSS, mobile app sync, OPML portability
- Gitea: lightweight GitHub alternative, webhook pipeline

Article count: 33 → 36. Open source section: 6 → 9.

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

SUMMARY.md

@@ -4,6 +4,9 @@
 * [Self-Hosting](02-selfhosting/index.md)
     * [Introduction](02-selfhosting/index.md)
 * [Open Source & Alternatives](03-opensource/index.md)
+    * [SearXNG: Private Self-Hosted Search](03-opensource/alternatives/searxng.md)
+    * [FreshRSS: Self-Hosted RSS Reader](03-opensource/alternatives/freshrss.md)
+    * [Gitea: Self-Hosted Git](03-opensource/alternatives/gitea.md)
     * [rmlint: Duplicate File Scanning](03-opensource/productivity/rmlint-duplicate-scanning.md)
     * [tmux: Persistent Terminal Sessions](03-opensource/dev-tools/tmux.md)
     * [screen: Simple Persistent Sessions](03-opensource/dev-tools/screen.md)