[DOC] Add missing weighted_routing nodes stats

[lukas-vlcek]

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The Nodes Stats documentation is missing weighted_routing metric group and its details.

I think the feature has been added in opensearch-project/OpenSearch#6041 and a high-level explanation can be found also here opensearch-project/OpenSearch#4735 (comment). Unfortunately, the documentation was not included.

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# Do a clean checkout of OpenSearch from git and run a single node:
./gradlew clean run

# After it is ready open a web browser of your choice and point it to nodes stats API
# using `weighted_routing` as a metric group filter.

open -a "Google Chrome"  "http://localhost:9200/_nodes/stats/weighted_routing?pretty&filter_path=nodes.*.weighted_routing"

We can see the following:

{
  "nodes" : {
    "usAVD2H5Sn6SaRaN-Z67Mg" : {
      "weighted_routing" : {
        "stats" : {
          "fail_open_count" : 0
        }
      }
    }
  }
}

PR link: #7245