chore: add gemini config (googleapis#2962)

Author: BenWhitehead

.gemini/config.yaml

@@ -0,0 +1,10 @@
+# https://developers.google.com/gemini-code-assist/docs/customize-gemini-behavior-github#custom-configuration
+have_fun: false
+code_review:
+  disable: false
+  comment_severity_threshold: HIGH
+  max_review_comments: -1
+  pull_request_opened:
+    help: false
+    summary: false
+    code_review: false