chore: protect docs/ from Fern regeneration

Add docs/ to .fernignore so the upcoming manually-written SDK
documentation is not overwritten by fern generate.

Prerequisite for PRD-003 (Python SDK docs).

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

Author: digitallysavvy

.fernignore

@@ -6,3 +6,6 @@ changelog.md
 
 # Wrapper layer - custom code that should not be overwritten
 src/agoraio/wrapper/
+
+# Documentation - managed manually, not generated by Fern
+docs/