[DOCS] Adds multi-bucket analysis to bucket overview

[lcawl]

Related to elastic/elasticsearch#34233

This PR adds the concept of multi-bucket analysis to the bucket overview in https://www.elastic.co/guide/en/elastic-stack-overview/master/ml-buckets.html

[sophiec20]

LGTM so far ... but I also think we should mention that you may see anomalies within the model bounds (if model plot is enabled as in the pic) ... this is because the blue shaded area is a simplified 2D representation of the single bucket model. This isn't just due to multi-bucket but it is more noticeable since. I think the blog may have worded it better.

[lcawl]

Thanks, I checked out the wording of the note in https://www.elastic.co/blog/changes-to-elastic-machine-learning-anomaly-scoring-in-6-5 and added a paragraph here.

When I refresh the ML getting started tutorial (in a separate PR), I'll also add that information there (e.g. where we discuss model bounds in https://www.elastic.co/guide/en/elastic-stack-overview/master/ml-gs-job1-analyze.html)

[sophiec20]

LGTM