Add documentation for area lights in 3D lights and shadows

[Calinou]

• Docs counterpart of Add area light examples to the 3D Lights and Shadows demo godot-demo-projects#1302.
• Requires Add Rectangular Area Light Source godot#108219 to be merged first (and classref to be synced for CI to pass here).

[Calinou]

Now that godotengine/godot#108219 was merged, this is now ready for review again 🙂

cc @mhilbrunner

[ArtyIF]

Shouldn't the new built-ins (LIGHT_AREA_DIFFUSE_MULTIPLIER, LIGHT_AREA_SPECULAR_MULTIPLIER, LIGHT_IS_AREA) be added to the light built-ins table?

[skyace65]

The class reference page for AreLight3D was added in the sync last week. Please rebase to fix the build issue.

[skyace65]

Thanks!