Add document explaining the schema of Cassandra executions table

[Shaddoll]

What changed?
Add document explaining the schema of Cassandra executions table

Why?
Improve documentation

How did you test it?

Potential risks

Release notes

Documentation Changes

[coveralls]

Pull Request Test Coverage Report for Build 018ef3ba-fc7e-4e84-8419-119ab3a582bc

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 34 unchanged lines in 10 files lost coverage.
• Overall coverage decreased (-0.002%) to 67.577%

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Files with Coverage Reduction	New Missed Lines	%
service/history/task/transfer_standby_task_executor.go	2	87.86%
common/task/weighted_round_robin_task_scheduler.go	2	88.56%
common/task/fifo_task_scheduler.go	2	83.51%
host/taskpoller.go	3	71.94%
service/history/queue/timer_queue_processor_base.go	3	77.82%
common/asyncworkflow/queue/consumer/default_consumer.go	3	92.13%
common/persistence/statsComputer.go	3	96.07%
common/task/parallel_task_processor.go	4	91.67%
service/matching/taskListManager.go	5	79.9%
service/history/execution/mutable_state_task_refresher.go	7	62.34%

Totals
Change from base Build 018ef375-14ca-473d-af52-3b92e8e756cd:	-0.002%
Covered Lines:	98959
Relevant Lines:	146438

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💛 - Coveralls