[Foundations] Improve Shadows documentation and DarkMode elevations

[JeremiasUX]

We don't provide DarkMode shadows and the shadows are not visible in DarkMode.

• https://spark.adevinta.com/1186e1705/p/39675a-elevation

• https://adevinta.slack.com/archives/C04LNCEMMUN/p1716819958409859

• [x] Improve the existing documentation based on RE work

• [x] Create DarkMode shadows

• [x] Update Foundations

• [x] Document in ZH

• [x] Share the improvement

[JeremiasUX]

New answers here: https://adevinta.slack.com/archives/C04LNCEMMUN/p1716819958409859

[JeremiasUX]

Spark Foundation branch can be found here:

• https://www.figma.com/design/ezqkDE3LqYrek9MvYnr4dR/branch/IhuHDOyPbBmqUTXw3T5PG9/Spark-Foundations?m=auto

Design exploration:

• https://www.figma.com/design/epxpKoREcESdwiXgqJPzZY/Test-Spark?node-id=1193-16781&t=SH8SyxlNu672kvND-0

Current solution - not perfect :

• Android uses the material shadow
• iOS uses white color in dark mode

[lauragautheron]

assigned to @sarah hattab