feat: Adds extension to argument parser's command configuration

Sources/CliDoc/Nodes/Usage.swift

@@ -0,0 +1,14 @@
+import CliDocCore
+
+public struct Usage<Content: TextNode>: TextNode {
+  @usableFromInline
+  let content: Content
+
+  public init(@TextBuilder content: () -> Content) {
+    self.content = content()
+  }
+
+  public var body: some TextNode {
+    content
+  }
+}