[DOC] Document Segment Replication Feature.

[mch2]

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With 2.3 we are looking to release an experimental version of the segment replication feature.
Design proposal - opensearch-project/OpenSearch#2229
RFC - opensearch-project/OpenSearch#1694 (comment)

As an experimental feature, it will be behind a feature flag that must be enabled on each node of a cluster in addition to setting the replication strategy during index creation.

To enable the feature flag:
For developers to enable with Gradle run:
Update run.gradle with:

testClusters {
  runTask {
    testDistribution = 'archive'
    if (numZones > 1) numberOfZones = numZones
    if (numNodes > 1) numberOfNodes = numNodes
    systemProperty 'opensearch.experimental.feature.replication_type.enabled', 'true'
  }
}

To enable on a node using a tarball:
option 1. Update config/jvm.options with:
-Dopensearch.experimental.feature.replication_type.enabled=true

option 2. include the flag in OPENSEARCH_JAVA_OPTS before ./bin/opensearch

OPENSEARCH_JAVA_OPTS="-Dopensearch.experimental.feature.replication_type.enabled=true" ./opensearch-3.0.0/bin/opensearch

option 3: Specify OPENSEARCH_JAVA_OPTS env var.

export OPENSEARCH_JAVA_OPTS="-Dopensearch.experimental.feature.replication_type.enabled=true"
./bin/opensearch

Setting replication strategy on the index:

curl -X PUT "localhost:9200/<index-name>?pretty" -H 'Content-Type: application/json' -d '{"settings" : { "index" : { "replication.type" : "SEGMENT" }}}'

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.