Add DocC page

Sources/ForceSimulation/ForceSimulation.docc/Creating2DAnd3DSimulations.md

@@ -0,0 +1,33 @@
+# Creating 2D and 3D simulations 
+
+
+## Overview
+
+You can simply create 2D or 3D simulations by using Simulation2D or Simulation3D:
+
+```swift
+import NDTree
+import ForceSimulation
+
+struct Node: Identifiable { ... }
+
+let nodeIds: [Node.ID] = ... 
+let links: [(Node.ID, Node.ID)] = ... 
+
+let sim = Simulation2D(nodeIds: nodeIds, alphaDecay: 0.01)
+sim.createManyBodyForce(strength: -12)
+sim.createLinkForce(links)
+sim.createCenterForce(center: [0, 0], strength: 0.4)
+sim.createCollideForce(radius: .constant(3))
+
+/// Force is ready to start! run `tick` to iterate the simulation.
+
+for i in 0..<120 {
+    sim.tick()
+    let positions = sim.nodePositions
+    /// Do something with the positions.
+}
+
+```
+
+See [Example](https://github.com/li3zhen1/Grape/tree/main/Examples/ForceDirectedGraphExample) for more details.

Sources/ForceSimulation/ForceSimulation.docc/Documentation.md

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+# ``ForceSimulation``
+
+Run force simulation on any dimension.
+
+## Overview
+
+ForceSimulation is a force simulation library that enables you to create any dimensional simulation with velocity Verlet integration.
+
+The basic concepts of simulations and forces can be found here: [Force simulations - D3](https://d3js.org/d3-force/simulation). 
+
+
+@Image(source: "ForceDirectedGraph.png", alt: "An example of 2D force directied graph.")
+
+
+
+## Topics
+
+
+### Creating a simulation
+
+* <doc:Creating2DAnd3DSimulations>
+* ``Simulation2D``
+* ``Simulation3D``
+
+* ``Simulation``
+
+### Creating forces in a simulation
+* ``CenterForce``
+* ``CollideForce``
+* ``LinkForce``
+* ``ManyBodyForce``
+* ``DirectionForce``
+* ``RadialForce``
+* ``ForceLike``

Sources/ForceSimulation/Simulation.swift

@@ -136,8 +136,10 @@ public final class Simulation<NodeID, V> where NodeID: Hashable, V: VectorLike,
 
 #if canImport(simd)
 
+    /// A 2D simulation running on `Double` and `simd_double2` types.
     public typealias Simulation2D<NodeID> = Simulation<NodeID, Vector2d> where NodeID: Hashable
 
+    /// A 3D simulation running on `Double` and `simd_double2` types.
     public typealias Simulation3D<NodeID> = Simulation<NodeID, Vector3d> where NodeID: Hashable
 
 #endif