Add DocC to SwiftSyntax

Author: kimdv

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.3
+// swift-tools-version:5.6
 
 import PackageDescription
 import Foundation
@@ -43,6 +43,9 @@ let package = Package(
     .library(name: "SwiftSyntaxBuilder", type: .static, targets: ["SwiftSyntaxBuilder"]),
     .executable(name: "SwiftSyntaxBuilderGeneration", targets: ["SwiftSyntaxBuilderGeneration"])
   ],
+  dependencies: [
+    .package(url: "https://github.com/apple/swift-docc-plugin", from: "1.0.0"),
+  ],
   targets: [
     .target(
       name: "_CSwiftSyntax",

Sources/SwiftSyntax/Documentation.docc/Documentation.md

@@ -0,0 +1,8 @@
+# ``SwiftSyntax``
+
+SwiftSyntax is a set of Swift bindings for the
+[libSyntax](https://github.com/apple/swift/tree/main/lib/Syntax) library. It
+allows Swift tools to parse, inspect, generate, and transform Swift source
+code.
+
+Its API is designed for performance-critical applications. It uses value types almost exclusively and aims to avoid existential conversions where possible.