chore: Add style guide to gemini.md #18744

Job	Run time
gapic-libraries-bom	18m 17s
bulk-filter	1m 7s
windows	2m 19s
enforcer	18m 28s
lint	2m 31s
generation-config-check	1m 45s
changes	5s
split-units (sdk-platform-java, 8)	4m 2s
split-units (sdk-platform-java, 25)	4m 18s
split-units (sdk-platform-java, 17)	4m 20s
split-units (sdk-platform-java, 11)	5m 7s
split-units (sdk-platform-java, 21)	4m 7s
units (8)	52s
units (11)	37s
units (25)	34s
units (21)	39s
units (17)	32s
	1h 9m 40s

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