Add SPI DocC support (#11)

Steps from https://blog.swiftpackageindex.com/posts/auto-generating-auto-hosting-and-auto-updating-docc-documentation/

.spi.yml

@@ -0,0 +1,4 @@
+version: 1
+builder:
+  configs:
+  - documentation_targets: [VaporRouting]

Package.swift

@@ -34,3 +34,10 @@ let package = Package(
     ),
   ]
 )
+
+#if swift(>=5.6)
+  // Add the documentation compiler plugin if possible
+  package.dependencies.append(
+    .package(url: "https://github.com/apple/swift-docc-plugin", from: "1.0.0")
+  )
+#endif