2D movement project can be replaced with a demo

It seems that the 2D movement project is used in the article more like a demo than like a starter template. So it should probably be removed and replaced with a demo project of a similar kind.

https://github.com/godotengine/godot-docs-project-starters/tree/master/src/2d_movement_starter
https://docs.godotengine.org/en/latest/tutorials/2d/2d_movement.html

Start by reviewing src/2d_movement_starter and the linked 2D movement article to understand how the project is used. Determine the scope of a similar demo replacement, then remove the starter project and add the replacement so the article points to the intended demo.

Written by the indexing model from the issue text.