Add metadata file

.matharchive.yaml

+Name: "GAP"
+Url: "https://gl.mathhub.info/ODK/GAP"
+Version: "4.10.2"
+Structure:
+  ExportPath: "gap"
+  ExportFiles:
+    - 'gaptypes.json'
+  MmtPath: "content/https..www.gap-system.org"
+System:
+  HomepageUrl: "https://www.gap-system.org"
+  DocumentationUrl: "https://www.gap-system.org/Manuals/doc/ref/chap0.html"
+    # TODO Not pinned to exact version
+    # TODO Link into documentation not possible
+  RepositoryUrl: "https://github.com/gap-system/gap"
+  RepositoryTag: "v4.10.2"
+  RepositoryPath: "lib"