docs: Cross-reference ProceduralSkyMaterial from related documentation

The ProceduralSkyMaterial page was generated and listed in the Helpers
module index, but none of the pages a reader starts from mentioned it.
Add links from SkyMaterial, ProceduralSkyTextureData, the
SceneEnvironment skyMaterial property and SkyMaterial background mode,
and the asset introduction, so the GPU-based alternative to
ProceduralSkyTextureData is discoverable.

Change-Id: Ife5f9faa802eee932b8c808f75b91dded09f7523
Reviewed-by: Christian Strømme <christian.stromme@qt.io>