Document `PostTick` and `PreTick` for Simulation

We should add a section to the Physics manual to show how to subscribe to the physics tick events in order to mimic what people are used to for FixedUpdate.

• This section should get the simulation
• subscribe to both PostTick and PreTick
• add some comments as to how they run

Example:

using Stride.Engine;
using Stride.Physics;

public class FixedUpdate : StartupScript
{
	public override void Start()
	{
		var sim = this.GetSimulation();
		sim.PreTick += Sim_PreTick;
		sim.PostTick += Sim_PostTick;
	}

	private void Sim_PreTick(Simulation sender, float tick)
	{
		// Run before the Simulation is updated
	}

	private void Sim_PostTick(Simulation sender, float tick)
	{
		// Run after the Simulation is updated
	}

	public override void Cancel()
	{
		var sim = this.GetSimulation();
		sim.PreTick -= Sim_PreTick;
		sim.PostTick -= Sim_PostTick;
	}
}

This should be a pretty simple manual page I think but we should also add it to the Stride for Unity® developers somewhere that would mention FixedUpdate from Unity.

more things to think about:

• The Bepu physics implementation does not have these events. This will be a Bullet physics doc.
• C# events kind of suck for performance apparently. Maybe we should give an example on how to make a single event call that can call multiple scripts with some sort of custom processor?
• Maybe for Bepu we can show how to use the SimulationUpdateComponent which works very similar to Unitys Monobehaviour with the Fixedupdate replaced by SimulationUpdate and AfterSimulationUpdate methods.

Start with the linked PhysicsScriptComponentExtensions API and the PostTick and PreTick definitions in Simulation.cs. Add a Physics manual section covering subscription, execution order, and cleanup, then cross-reference it from the Stride for Unity developers page near FixedUpdate. Document the Bullet-only limitation and the Bepu alternatives mentioned in the issue.