docs-bug(THEMING): Theme config migration since v18

### Documentation Feedback

In our project we started using Material 3 theming since Angular 18.
We used for that the following mixins mat.define-theme, mat.define-light-theme, or mat.define-dark-theme.
In usage like:

```scss
@use '@angular/material' as mat;

$theme: mat.define-theme((
color: (
theme-type: dark,
primary: mat.$violet-palette,
),
typography: (
brand-family: 'Comic Sans',
bold-weight: 900
),
density: (
scale: -1
)
));
```

But since Angular 19 and now Angular 20, we have in the documentation [Here](https://material.angular.dev/guide/theming) a comment indicating to still have a look to v18 documentation.

`If your application depends on a version before v19, or if your application's theme is applied using a theme config created with mat.define-theme, mat.define-light-theme, or mat.define-dark-theme, then you can refer to the theming guides at [v18.material.angular.dev/guides](https://v18.material.angular.dev/guides).`

Regarding that, several questions/clarifications will help a lot:
- the "old way" used since V18 is going to be deprecated ?
- we follow all release notes since v18, but didn't see anything explaining how to migrate to the new functions ?
- Is there a schematic available for that migration ?

Thanks for your help

### Affected documentation page

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