Add documentation custom style layers

[louwers]

Follow-up to #3154 from @christian-boks.

• Moves the documentation comments to the header so they show up in the documentation. I think Mapbox decided to not make this public because they didn't want to officially support it.
• Makes some tweaks to the custom style layer so the triangle stays in place when you rotate the map.
• Adds documentation page for custom style layer.

Please help me check if ChatGPT did a good job. 😄

[github-actions]

Bloaty Results (iOS) 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0% +2.29Ki  [ = ]       0    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results-ios/pr-3165-compared-to-main.txt