Add docs for index-rollover feature

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

Add documentation for https://github.com/jaegertracing/jaeger/pull/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 https://github.com/jaegertracing/jaeger/pull/2935#issuecomment-821046608:

- 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 https://github.com/jaegertracing/jaeger/pull/2935#issuecomment-821127032.
- Time-based rollover only (day, hour)
- Simple to setup
- Requires self-managed cleanup of old indices

Start at the Elasticsearch rollover section linked in the issue and review PR #2935 plus its linked comments for the three index-management options. Document each option's supported rollover behavior, requirements, and tradeoffs; it is done when the page clearly distinguishes manual scripts, Elasticsearch ILM, and time-based self-managed cleanup.

Written by the indexing model from the issue text.