Explain that "position" is an inbuilt variable explicitly in lesson three of the Getting Started documentation

**Your Godot version:**
4.3

**Issue description:**
In this lesson there is a section where you are assigning new value to a position variable for the player. This might be a bit confusing. Mentioning the existence of inbuilt variables seems like a good learning opportunity as well, so my suggestion would be adding some explicit mention as a small quality of life feature in this lesson.

**URL to the documentation page (if already existing):**
https://github.com/godotengine/godot-docs/blob/c5babea4bdc77893ecdbebbac5e39f9ff50755a1/getting_started/first_2d_game/03.coding_the_player.rst?plain=1#L226