Docs: Mark variable_width_histogram experimental (#58574)

We're tracking this aggregation's experimental-progress in #58573. We'd
like a little time to be able to make backwards incompatible changes to
the aggregation because we're not 100% sure about the request and
response format yet.

Author: nik9000

docs/reference/aggregations/bucket.asciidoc

@@ -68,4 +68,6 @@ include::bucket/significanttext-aggregation.asciidoc[]
 
 include::bucket/terms-aggregation.asciidoc[]
 
+include::bucket/variablewidthhistogram-aggregation.asciidoc[]
+
 include::bucket/range-field-note.asciidoc[]

docs/reference/aggregations/bucket/variablewidthhistogram-aggregation.asciidoc

@@ -1,6 +1,8 @@
 [[search-aggregations-bucket-variablewidthhistogram-aggregation]]
 === Variable Width Histogram Aggregation
 
+experimental::["We're evaluating the request and response format for this new aggregation.",https://github.com/elastic/elasticsearch/issues/58573]
+
 This is a multi-bucket aggregation similar to <<search-aggregations-bucket-histogram-aggregation>>.
 However, the width of each bucket is not specified. Rather, a target number of buckets is provided and bucket intervals
 are dynamically determined based on the document distribution. This is done using a simple one-pass document clustering algorithm