docs: organize test index into categories #5945

Job	Run time
sample_operators_tests (sample-operators/mysql-schema)	2m 43s
sample_operators_tests (sample-operators/tomcat-operator)	2m 50s
sample_operators_tests (sample-operators/leader-election)	3m 19s
sample_operators_tests (sample-operators/webpage)	3m 32s
	12m 24s

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