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>

.env

@@ -0,0 +1,22 @@
+# ===----------------------------------------------------------------------===
+# 
+# This source file is part of the MarvelService open source project
+# 
+# Copyright (c) 2025 Röck+Cöde VoF. and the MarvelService project authors
+# Licensed under the EUPL 1.2 or later.
+# 
+# See LICENSE for license information
+# See CONTRIBUTORS for the list of MarvelService project authors
+# 
+# ===----------------------------------------------------------------------===
+
+# --- DOCUMENTATION ---
+
+DOCC_ARCHIVE_OUTPUT=./${SPM_LIBRARY_TARGET}.doccarchive
+DOCC_GITHUB_OUTPUT=./docs
+DOCC_GITHUB_BASE_PATH=marvel-service
+DOCC_PREVIEW_URL=http://localhost:8080/documentation/marvelservice
+
+# -- SWIFT PACKAGE MANAGER ---
+
+SPM_LIBRARY_TARGET=MarvelService