docs(material/theming): rewrite theming guide for @use

This change completely rewrites the theming guide to be more complete, correct, and concise.

Summary of changes:
* Explain the concepts of palettes, hues, and themes.
* Ensure all content is conceptual and not task-based.
* Use the new Sass `@use` API introduced in #22173
* Document usage of the theming system with shadow dom
* Document the fact that CSS overrides are strongly discouraged
* Be more explicit about which palettes we provide
* Consolidate on a single approach to including theme CSS in an app (assuming Angular CLI)

I will send follow-up PRs for the typography, theming-your-components, and customizing-component-styles guides.

Author: jelbourn