Lack of documentation on shape-casting & low-level physics interaction

**Your Godot version:** 3.3.2

**Issue description:**
No documentation of shape-casting and other low-level physics interaction.

There isn't **in the whole internet** any kind of material (docs, tutorials, etc) about shape-casting (`PhysicsDirectSpaceState` 2D/3D). In fact it means that **it is unknown by the bast majority** of the users.
The few that can found is 2D, and too limited.

So how newcomers will implement cool things like *stair-stepping*, *edge-friction*, complex movement engagement (ie TombRaider-like climbing), non-rudimentary navigation along walls and ceilings (ie creature climb), and AI spatial queries, etc.

Its true that advanced users can dig into it and get things working, but for most of the novel game-developers and/or programmers, it can become a roadblock (as recently talking with a couple).

Currently all stuff involving it:
- PhysicsServer
- World
- DirectStates
- ShapeQueries
- PhysicsMaterial
- advanced PhysicsBody sutff
The most remain behind the veil realm of the shadows.

To favor that more virtuous games get shipped, good material about would be determinant, addressing typical uses, typical doubts, mistakes and misunderstands, and a general overview to dig onto it.

Giving the tools for do advanced stuff will lead for more advanced stuff coming out.