This commit adds a new bg_count field to the REST response of
SignificantTerms aggregations. Similarly to the bg_count that already
exists in significant terms buckets, this new bg_count field is set at
the aggregation level and is populated with the superset size value.
Added missing CONSOLE scripts to documentation for sampler and diversified_sampler aggs.
Includes new StackOverflow index setup in build.gradle
Closes#22746
* Formatting tweaks
Tanguy Leroux
markharwood
Colin Goodheart-Smithe
Zachary Tong