Add CumulativeCard pipeline agg to pipeline index (#46279)

The Cumulative Cardinality docs weren't linked
from the pipeline index page

docs/reference/aggregations/pipeline.asciidoc

@@ -281,6 +281,7 @@ include::pipeline/percentiles-bucket-aggregation.asciidoc[]
 include::pipeline/movavg-aggregation.asciidoc[]
 include::pipeline/movfn-aggregation.asciidoc[]
 include::pipeline/cumulative-sum-aggregation.asciidoc[]
+include::pipeline/cumulative-cardinality-aggregation.asciidoc[]
 include::pipeline/bucket-script-aggregation.asciidoc[]
 include::pipeline/bucket-selector-aggregation.asciidoc[]
 include::pipeline/bucket-sort-aggregation.asciidoc[]