docs-bug(elevation): No documentation for how to change shadow colour for elevation

### Documentation Feedback

The current documentation does not provide sufficient documentation for how to change the shadow colour (possibly more relevant with dark themes).

I have found the [original commits for the feature](https://github.com/angular/components/issues/11343), so I know it is achievable, but I could not find enough documentation to implement it myself. My attempts can be seen at [this Stack Blitz](https://stackblitz.com/edit/material-shadow-demo?file=src%2Fglobal_styles.scss).

I'm selfishly just wanting to know how to implement the feature, but I'm also willing to update the documentation if that would be of assistance.

Thank you all for your work - Angular Material is great.

### Affected documentation page

https://material.angular.io/guide/elevation#overridable-elevation

Beginne mit der betroffenen Seite unter material.angular.io/guide/elevation#overridable-elevation und prüfe das ursprüngliche Feature-Issue 11343. Verwende den verlinkten StackBlitz, um die fehlenden Hinweise zur Schattenfarbe zu verstehen, aktualisiere anschließend die Elevation-Dokumentation mit reproduzierbaren Anweisungen und verifiziere, dass der Anwendungsfall für das dunkle Theme abgedeckt ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.