Updating and Improving Documentation for Gridmap Editor

4.4-master-branch

Now that this PR (https://github.com/godotengine/godot/pull/96922) was merged, it is time to write a little bit of documentation on the Gridmap bottom panel, and all the changes that have happened.

I think it will also be a nice opportunity to:
- give walkthrough the new UI and shortcuts
- presenting each tools
- give an example of how to organise the scene tree when making a level (where do I put my interactive scenes?)
- clear any confusion on the scope of Gridmap (which is to place static background meshes)

Let me know what you think.