Add page "custom drawing in 3D"

**Your Godot version:** 4.4.beta1

**Issue description:** i am prototyping procedural 3D visuals, so i need to use custom drawing in 3D to create procedural 3D visuals but there is no "custom drawing in 3D" documentation page.
if there is documentation about cusom drawing in 3D, i can't find it because i can't think of another name that such documentation could have except "custom drawing in 3D"

**URL to the documentation page (if already existing):**
the documentation page does not exist. here are the [search results](https://docs.godotengine.org/en/latest/search.html?q=custom+drawing+in+3D&check_keywords=yes&area=default) which should find the page "custom drawing in 3D" but they did not