Add docs for index-rollover feature

[albertteoh]

Requirement - what kind of business use case are you trying to solve?

Add documentation for jaegertracing/jaeger#2935

Problem - what in Jaeger blocks you from solving the requirement?

It can be confusing with three different mechanisms for elasticsearch index management.

Proposal - what do you suggest to solve the problem or improve the existing situation?

Add documentation to https://www.jaegertracing.io/docs/deployment/#elasticsearch-rollover that clearly states the options available for index management and their tradeoffs, perhaps something like jaegertracing/jaeger#2935 (comment):

• Manual index management with esRollover/cleaner scripts:
  • Time, size and doc count rollover support
  • Tried and tested.
  • Requires setup and maintenance of cron jobs.
• Automated index management with Elasticsearch's ILM.
  • Time, size and doc count rollover support
  • Requires ES 7+
  • Experimental
• Time-based automated rollover with self-managed cleanup of indices (i.e. ILM) as suggested in [abandoned][es] Add index rollover mode that can choose day and hour jaeger#2935 (comment).
  • Time-based rollover only (day, hour)
  • Simple to setup
  • Requires self-managed cleanup of old indices