Add manual page for AtlasTexture

**Your Godot version:**
3.2+

**Issue description:**
We currently don't seem to have a page that covers usage of AtlasTexture as re-implemented in Godot.

Most users seem to use https://godotengine.org/article/atlas-support-returns-godot-3-2 as a reference to know how to create an AtlasTexture from a set of sprites. This should be properly integrated in the official docs.

The documentation should likely cover both how to create/import an AtlasTexture from a set of sprites, and how to create an AtlasTexture from a spritesheet in the Inspector / Texture region editor. And then how to use them practically.

**URL to the documentation page (if already existing):**
n/a