docs: Fix list rendering in Map.add_layer API Docs (#952)

Before:

<img width="1394" height="316" alt="image"
src="https://github.com/user-attachments/assets/dc30df82-dad1-4b0e-b04f-fff8efa9e57c"
/>

After:

<img width="725" height="361" alt="image"
src="https://github.com/user-attachments/assets/ce90d3b7-318d-4f27-9111-42f59988ef98"
/>

Author: kylebarron

lonboard/_map.py

@@ -428,6 +428,7 @@ def add_layer(
 
         Args:
             layers: New layers to add to the map. This can be:
+
                 - a layer instance
                 - a list or tuple of layer instances
                 - another `Map` instance, in which case its layers will be added to this