[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>

Package.resolved

@@ -0,0 +1,23 @@
+{
+  "pins" : [
+    {
+      "identity" : "swift-docc-plugin",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/apple/swift-docc-plugin",
+      "state" : {
+        "revision" : "26ac5758409154cc448d7ab82389c520fa8a8247",
+        "version" : "1.3.0"
+      }
+    },
+    {
+      "identity" : "swift-docc-symbolkit",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/apple/swift-docc-symbolkit",
+      "state" : {
+        "revision" : "b45d1f2ed151d057b54504d653e0da5552844e34",
+        "version" : "1.0.0"
+      }
+    }
+  ],
+  "version" : 2
+}