Prepare for DocC (#1467)

# Motivation
We wanna use DocC for publishing docs.

# Modification
Setup our `Package.swift`  manifests to enable proper doc generation.

# Result
We can now use DocC to generate docs.

Author: FranzBusch

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.4
+// swift-tools-version:5.6
 /*
  * Copyright 2017, gRPC Authors All rights reserved.
  *
@@ -67,6 +67,10 @@ let packageDependencies: [Package.Dependency] = [
     url: "https://github.com/apple/swift-argument-parser.git",
     from: argumentParserMinimumVersion
   ),
+  .package(
+    url: "https://github.com/apple/swift-docc-plugin",
+    from: "1.0.0"
+  ),
 ].appending(
   .package(
     url: "https://github.com/apple/swift-nio-ssl.git",