docs-bug(theming): Add back explanation about some prebuilt themes being M2 and some M3

[JWess]

Documentation Feedback

This PR added a very helpful section to the theming doc that explained which prebuilt themes are M2 and which are M3. Unfortunately, the very helpful section below was recently removed when the docs were overhauled. After updating to the latest versions of Angular Material, I had no idea why my theming still looked so different than what's shown on the Angular Material website. As it turns out, it's because some prebuilt themes are Material 2 themes. I only figured that out when I found the above PR.

I think it should be added back in.

Affected documentation page

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