Add design doc for workflow shadowing

[yycptt]

What changed?
Add design doc for workflow shadowing

Why?

How did you test it?

Potential risks

Release notes

Documentation Changes

[coveralls]

Pull Request Test Coverage Report for Build 047dee56-dc02-4f3e-96e2-ea7cf14ee2c9

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 47 unchanged lines in 8 files lost coverage.
• Overall coverage increased (+0.05%) to 59.89%

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Files with Coverage Reduction	New Missed Lines	%
host/signalworkflowTest.go	1	95.53%
common/persistence/executionStore.go	2	73.86%
common/persistence/statsComputer.go	2	95.64%
common/task/fifoTaskScheduler.go	2	84.54%
service/history/queue/timer_queue_processor.go	2	58.77%
service/history/shard/context.go	8	61.88%
service/history/execution/context.go	14	66.78%
common/persistence/sql/sqlExecutionManager.go	16	58.48%

Totals
Change from base Build 62117386-7edd-49d6-8ecf-8100a77243f2:	0.05%
Covered Lines:	88140
Relevant Lines:	147170

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💛 - Coveralls