Add a 'Hello, World' tutorial (#2023)

Motivation:

Tutorials are a great way to introduce new users to the library.

Modifications:

- Add a 'hello, world' tutorial

Result:

Easier to get started

Author: glbrntt

scripts/license-check.sh

@@ -107,6 +107,7 @@ check_copyright_headers() {
     ! -name '*.grpc.swift' \
     ! -name 'LinuxMain.swift' \
     ! -name 'XCTestManifests.swift' \
+    ! -path './Sources/GRPCCore/Documentation.docc/*' \
     ! -path './FuzzTesting/.build/*' \
     ! -path './Performance/QPSBenchmark/.build/*' \
     ! -path './Performance/Benchmarks/.build/*' \