[Enhancement] DocC documentation support (#23)

This PR contains the work done to add DocC support to all the libraries included in this package, as requested by issue #9.

To provide further details about the work done:
- [x] added the DocC plugin package as a dependency
- [x] improved the naming of the targets in the Package file
- [x] updated the README file
- [x] implemented the Xcode and Github pages documentation generation targets in the Makefile file
- [x] generated both Xcode and Github pages documentation

Reviewed-on: #23
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

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+
+  Copyright (c) 2021 Apple Inc. and the Swift project authors
+  Licensed under Apache License v2.0 with Runtime Library Exception
+
+  See https://swift.org/LICENSE.txt for license information
+  See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+-->
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Documents/SwiftLibsPersistence.doccarchive/img/deprecated-icon.015b4f17.svg

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+<!--
+  This source file is part of the Swift.org open source project
+
+  Copyright (c) 2021 Apple Inc. and the Swift project authors
+  Licensed under Apache License v2.0 with Runtime Library Exception
+
+  See https://swift.org/LICENSE.txt for license information
+  See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+-->
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Documents/SwiftLibsPersistence.doccarchive/img/modified-icon.f496e73d.svg

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+<!--
+  This source file is part of the Swift.org open source project
+
+  Copyright (c) 2021 Apple Inc. and the Swift project authors
+  Licensed under Apache License v2.0 with Runtime Library Exception
+
+  See https://swift.org/LICENSE.txt for license information
+  See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+-->
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