docs: Add licensing/copyright data to GVariant specification

The licensing for the original GVariant specification was not specified
in the original PDF.

However, CC-BY-SA-3.0 has been agreed by Allison, the sole copyright
holder, here:

https://gitlab.gnome.org/Teams/documentation/developer-www/-/merge_requests/108/#note_1586866

The diagrams were redrawn by me, so their licensing/copyright status is
clear.

Tested with `reuse lint` to ensure the data is machine-readable.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

docs/reference/glib/gvariant-integer-and-string-structure.svg

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+   xmlns:svg="http://www.w3.org/2000/svg"
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+   xmlns:cc="http://creativecommons.org/ns#"
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@@ -25,7 +28,7 @@
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@@ -322,4 +325,34 @@
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+      <cc:Work
+         rdf:about="">
+        <dc:rights>
+          <cc:Agent>
+            <dc:title>Copyright © 2022 Philip Withnall</dc:title>
+          </cc:Agent>
+        </dc:rights>
+        <cc:license
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 </svg>