Update C++ style guide reference

The [contributing](https://github.com/google/bigwheels/blob/main/CONTRIBUTING.md#for-developers-contributing-a-patch) page advises contributors to make sure that the code "follows the [Google C++ Style Guide](https://google.github.io/styleguide/cppguide.html)." We currently don't follow the Google style guide. We should probably remove the reference and document the style guide we want to follow, and update .clang-format if necessary