[DOCS] Identify places in ML docs to add visuals

## Goal

This issue is for identifying places in ML docs where we can add visuals to convey and digest information easier.
Add the page name, URL, and define the position on the page where the visual should be added. Optionally describe in a few words what kind of visual is needed or add an example image.

- [ ] Change the workflow diagram in [Introduction to supervised learning](https://www.elastic.co/guide/en/machine-learning/master/ml-dfa-overview.html#ml-supervised-workflow)
- [ ] [Finding outliers](https://www.elastic.co/guide/en/machine-learning/master/ml-dfa-finding-outliers.html): after the first paragraph.
- [x] [Predicting classes with classification](https://www.elastic.co/guide/en/machine-learning/master/ml-dfa-classification.html): after the first paragraph that defines what classification analysis is.
- [ ] [Predicting numerical values with regression](https://www.elastic.co/guide/en/machine-learning/master/ml-dfa-regression.html): after the second paragraph.