chore: Add style guide to gemini.md #704

Job	Run time
filter	59s
build(8) for showcase	9m 35s
build(21) except self-service clients	12m 20s
build(25) except self-service clients	12m 22s
build (17)	13m 43s
build(8) except for gapic-generator-java	4m 0s
build (11)	13m 50s
showcase-clirr	5m 15s
build(8) for gapic-generator-java	11m 9s
unmanaged_dependency_check	10m 43s
showcase (17)	10m 30s
dependency_analyzer_unit_test	1m 18s
showcase (21)	10m 50s
gapic-generator-java-bom	2m 15s
showcase (11)	10m 49s
showcase (25)	10m 10s
	2h 19m 48s

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