chore: Add style guide to gemini.md #17341

Job	Run time
root-pom	1m 33s
gapic-bom	1m 33s
generate-readme	1m 5s
gitignore_correctness	1m 1s
owlbot-py	1m 8s
group_id_check_for_maps_libraries	7m 51s
owlbot-yaml	1m 1s
package_name_check	1m 0s
	16m 12s

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