* add indexer api design doc
* remove some api & add explain info for tiered storage
* rename tired storage level name
* add overview architecture
* define the input of indexer kvevent
* add example for indexer api output
* fix some issue from code reviews
* add explain for different medium
* delete the confusing note
* add more Introduction for indexer output value
* change some code based on the code review
* use medium name directly
* fix some format problem
* support multi-tenant & cache salt and rename engine_name
yejj