[Doc] documented the gml file

Documentation/ProjectFile/gml/surfaces/surface/i_surface.md

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+Defines a single triangulated surface.
+The surface is given as the set of all triangles it comprises.

Documentation/ProjectFile/gml/t_name.md

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+Name of the geometrical set, i.e., the entirety of all geometries defined in
+this file.
+Each individual geometry additionally has an own name, which has to be unique in
+the geometrical set.