Library setup (#1)

This PR contains the work done to setup the library:
* Defined products, dependencies, and targets for the `Package` file;
* Restructured the folder structure of the library and test targets to start development work;
* Created the `DocC` documentation catalog in the library target;
* Added the `CONTRIBUTORS` and `LICENSE` files;
* Added `README` file.

Reviewed-on: #1
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Sources/MarvelService/Catalogs/MarvelService.docc/Library.md

@@ -0,0 +1,13 @@
+# ``MarvelService``
+
+<!--@START_MENU_TOKEN@-->Summary<!--@END_MENU_TOKEN@-->
+
+## Overview
+
+<!--@START_MENU_TOKEN@-->Text<!--@END_MENU_TOKEN@-->
+
+## Topics
+
+### <!--@START_MENU_TOKEN@-->Group<!--@END_MENU_TOKEN@-->
+
+- <!--@START_MENU_TOKEN@-->``Symbol``<!--@END_MENU_TOKEN@-->

Sources/MarvelService/Public/marvel_service.swift

@@ -0,0 +1,2 @@
+// The Swift Programming Language
+// https://docs.swift.org/swift-book