Write a documentation page on creating good-looking 3D scenes

See also https://github.com/godotengine/godot-docs/issues/5121.

Looking at various social media, I keep seeing a lot of 3D scenes that don't look great and could easily be made to look significantly better with a little more work. For instance, when anisotropic filtering on textures isn't enabled in a desktop game (when it should almost always be in 2021).

In the long term, this relatively "amateur" appearance tends to give a bad image of Godot's 3D capabilities. This image may persist after 4.0 if people don't step up their 3D scene setup skills :slightly_smiling_face:

However, I'm worried about this page not being general-purpose enough for the Godot documentation. There's a lot of engine-agnostic resources on creating good-looking 3D scenes, but also a lot of good Unity/Unreal Engine-oriented resources that Godot users may shy away from.

This would be similar to the [Performance](https://docs.godotengine.org/en/latest/tutorials/performance/index.html) section we have, except that it'd be focusing on improving visuals instead. Sometimes, there would be performance tradeoffs involved in improving the visuals, but not always.