[DOC] Add Indices Request Cache related docs and their settings

[kiranprakash154]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request and all versions that are affected.
I observed in #6300 that there is no documentation for Indices Request Cache and other caching in general.

Below are the new settings added in Indices Request Cache introduced in opensearch-project/OpenSearch#12625 and registered in opensearch-project/OpenSearch#12941.

IndicesRequestCache.INDICES_REQUEST_CACHE_CLEANUP_INTERVAL_SETTING,
IndicesRequestCache.INDICES_REQUEST_CACHE_STALENESS_THRESHOLD_SETTING,

The above settings do not belong in the search plugins and hence documentation has to be created around Caching in general or atleast Indices Request Cache and the above new settings and the old settings like indices.cache.cleanup_interval
or indices.requests.cache.size

[kiranprakash154]

can piggy back @sgup432's PR - #7052

[kiranprakash154]

@kkhatua FYR this is the issue i have created to add docs for IRC in general