[DOC] Document behavior of new index.merge.policy setting

[rishabhmaurya]

Is your feature request related to a problem? Please describe.
Yes. A new index setting (index.merge.policy) is introduced to configure the index merge policy (tiered and log_byte_size) for individual index. Also, a new cluster setting (indices.time_series_index.default_index_merge_policy) is introduced to set the default merge policy for index containing @timestamp field. Both the settings defaults are currently set as tiered.
Both settings are static.

Related to - opensearch-project/OpenSearch#9992

Describe the solution you'd like
Document this behavior here - https://opensearch.org/docs/latest/im-plugin/index-settings/ for index setting
and here for cluster setting - https://opensearch.org/docs/latest/api-reference/cluster-api/cluster-settings/

Describe alternatives you've considered
None

Additional context
Add any other context or screenshots about the feature request here.

[reta]

FYI, this is scheduled for 2.11, thank you.

[rishabhmaurya]

PR - #5137

I'm planning to write a blog post with more details once these concerns are addressed