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 angular#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

guides/getting-started.md

@@ -20,7 +20,7 @@ The `ng add` command will install Angular Material, the [Component Dev Kit (CDK)
 
 1. Choose a prebuilt theme name, or "custom" for a custom theme:
 
-   You can choose from [prebuilt material design themes](https://material.angular.io/guide/theming#using-a-pre-built-theme) or set up an extensible [custom theme](https://material.angular.io/guide/theming#defining-a-custom-theme).
+   You can choose from [prebuilt material design themes](https://material.angular.io/guide/theming#using-a-pre-built-theme) or set up an extensible [custom theme](https://material.angular.io/guide/theming#defining-a-theme).
 
 2. Set up global Angular Material typography styles: