Link rare_terms docs from index page (#43882)

Docs for rare_terms were added in #35718, but neglected to
link it from the bucket index page

docs/reference/aggregations/bucket.asciidoc

@@ -55,6 +55,8 @@ include::bucket/parent-aggregation.asciidoc[]
 
 include::bucket/range-aggregation.asciidoc[]
 
+include::bucket/rare-terms-aggregation.asciidoc[]
+
 include::bucket/reverse-nested-aggregation.asciidoc[]
 
 include::bucket/sampler-aggregation.asciidoc[]