Add Gemini code review config (#12725)

Copied configuration from grpc-go for default code review settings from
grpc/grpc-go#8873

Author: sauravzg

.gemini/config.yaml

@@ -0,0 +1,13 @@
+have_fun: false
+memory_config:
+  disabled: false
+code_review:
+  disable: false
+  comment_severity_threshold: MEDIUM
+  max_review_comments: -1
+  pull_request_opened:
+    help: false
+    summary: false
+    code_review: false
+    include_drafts: false
+ignore_patterns: []