[DOCS] Added Data Frames subsection to ML section.

[szabosteve]

This PR adds a Data Frames overview subsection to the ML section.

[pheyos]

Would this documentation be the right place to mention the needed permissions around the whole data frame topic? Asking, because I currently can't find it anywhere and users might think machine_learning_admin might be the required role (since data frames are located under ML), which is not the case.

[lcawl]

@pheyos We typically add that type of information in the API reference material, but you're right I don't see it in https://www.elastic.co/guide/en/elasticsearch/reference/master/put-data-frame-transform.html for example. I'll follow-up and add it there and potentially in the (upcoming) configuration example too.

[pheyos]

thanks @lcawl !

[lcawl]

I added some comments, but otherwise LGTM and builds successfully