docs(material/theming): add section for overrides api to theming.md

[wagnermaciel]

Documentation Feedback

Add a section to theming.md to describe the 3 new override APIs. The section should describe:

1. The benefits of using these APIs.
2. How to use them.
3. What use case each is best suited for.

This information is already outlined in go/angular:mat-theming-overrides-api.

Affected documentation page

https://material.angular.io/guide/theming