Add DocC to SwiftSyntax

Author: kimdv

.spi.yml

@@ -0,0 +1,7 @@
+version: 1
+builder:
+  configs:
+    - platform: macos
+      documentation_targets: [SwiftSyntax, SwiftSyntaxBuilder, SwiftSyntaxParser]
+    - platform: linux
+      documentation_targets: [SwiftSyntax, SwiftSyntaxBuilder, SwiftSyntaxParser]

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.3
+// swift-tools-version:5.6
 
 import PackageDescription
 import Foundation
@@ -94,11 +94,11 @@ let package = Package(
       name: "_SwiftSyntaxTestSupport",
       dependencies: ["SwiftSyntax"]
     ),
-    .target(
+    .executableTarget(
       name: "lit-test-helper",
       dependencies: ["SwiftSyntax", "SwiftSyntaxParser"]
     ),
-    .target(
+    .executableTarget(
         name: "SwiftSyntaxBuilderGeneration",
         dependencies: ["SwiftSyntaxBuilder"],
         exclude: [

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.