chore: Add style guide to gemini.md #17340

Job	Run time
owlbot-yaml	1m 3s
gapic-bom	1m 42s
generate-readme	1m 8s
group_id_check_for_maps_libraries	7m 42s
gitignore_correctness	1m 2s
owlbot-py	1m 9s
package_name_check	1m 1s
root-pom	1m 42s
	16m 29s

Provide feedback