Add docs for the rest of classes

Sources/PNDependencyGraph/PNCompilationError.swift

@@ -0,0 +1,6 @@
+import Foundation
+
+public enum PNCompilationError: Error {
+    /// Indicates that the graph contains a cycle and cannot be compiled.
+    case cycle
+}

Sources/PNDependencyGraph/PNCompiledGraph.swift

@@ -1,5 +1,6 @@
 import Foundation
 
+/// A compiled dependency graph.
 public final class PNCompiledGraph {
     let nodes: [PNNode]
     init(nodes newNodes: [PNNode]) {

Sources/PNDependencyGraph/PNDependencyGraph.swift

@@ -1,27 +1,46 @@
 import Foundation
 
-public enum PNCompilationError: Error {
-    case cycle
-}
-
+/// The main class for building and compiling dependency graphs.
+///
+/// This class provides methods to add nodes, define dependencies between them,
+/// and compile the graph into a topologically sorted order.
 public final class PNGraph {
+    /// A dictionary of all nodes in the graph, keyed by their identifiers.
     private var nodes: [String: PNNode] = [:]
 
+    /// Adds a new node to the graph with the specified identifier.
+    ///
+    /// If a node with this identifier already exists, it is returned instead of creating a new one.
+    ///
+    /// - Parameter identifier: A unique identifier for the node.
+    /// - Returns: The node associated with the provided identifier.
     public func add(identifier: String) -> PNNode {
         if let existing = nodes[identifier] { return existing }
         let node = PNNode(identifier: identifier)
         nodes[identifier] = node
         return node
     }
 
+    /// Initializes an empty dependency graph.
     public init() {
 
     }
 
+    /// Retrieves a node from the graph by its identifier.
+    ///
+    /// - Parameter identifier: The identifier of the node to retrieve.
+    /// - Returns: The node with the specified identifier, or `nil` if not found.
     public func get(identifier: String) -> PNNode? {
         nodes[identifier]
     }
 
+    /// Compiles the dependency graph into a topologically sorted order.
+    ///
+    /// This method performs a depth-first search to detect cycles and generate an execution order.
+    /// If a cycle is detected, it throws `PNCompilationError.cycle`.
+    ///
+    /// - Returns: A compiled graph containing nodes in topological order.
+    /// - Throws: `PNCompilationError.cycle` if the graph contains a cycle.
     public func compile() throws -> PNCompiledGraph {
         var visited: Set<String> = []
         var visiting: Set<String> = []