chore: Add style guide to gemini.md #690

Job	Run time
filter	1m 0s
dependency_analyzer_unit_test	1m 17s
build(8) except for gapic-generator-java	4m 5s
showcase-clirr	5m 23s
build(21) except self-service clients	12m 45s
build(8) for showcase	10m 2s
build(25) except self-service clients	12m 29s
gapic-generator-java-bom	2m 18s
build (11)	13m 59s
build (17)	13m 25s
build(8) for gapic-generator-java	11m 16s
showcase (21)	10m 8s
showcase (17)	10m 54s
unmanaged_dependency_check	6m 52s
showcase (25)	10m 17s
showcase (11)	10m 18s
	2h 16m 28s

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