chore: update gemini styleguide (#15758)

Author: scotthart

.gemini/styleguide.md

@@ -1,5 +1,16 @@
 This repository mostly follows the Google C++ style guide found at
 https://google.github.io/styleguide/cppguide.html.
 
-Notably, this repository only allows language features and libraries found in
-C++14.
+This repository differs from the Google C++ style guide in the following ways:
+
+- supports C++14 as its minimum C++ standard version.
+- lists local includes first, then system includes.
+- uses std::mutex instead of absl::mutex.
+- uses google::cloud::future and google::cloud::promise instead of std::future
+  and std::promise.
+- uses google::cloud::Status and google::cloud::StatusOr instead of absl::Status
+  and absl::StatusOr.
+- prefers to factor out duplicated code if it appears 3 or more times in
+  non-test files.
+- encourages duplication of salient setup and expectations in test cases to
+  increase readability.