docs: add migration guide to nav

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

Author: domenkozar

docs/src/guides/.nav.yml

@@ -1,4 +1,5 @@
 nav:
+  - Migrating to 2.0: migrating-to-2.0.md
   - Monorepo: monorepo.md
   - Using with Flakes: using-with-flakes.md
   - Using with flake.parts: using-with-flake-parts.md