[Doc] documented the gml file

Documentation/ProjectFile/gml/c_gml.md

+In gml files geometries, i.e., points, polylines, and triangulated surfaces, are
+defined.
+These geometries can be used in project files, e.g., to set boundary conditions.

Documentation/ProjectFile/gml/points/i_points.md

-\ogs_missing_documentation
+Contains point definitions.

Documentation/ProjectFile/gml/points/point/a_id.md

-\ogs_missing_documentation
+ID of the point. It is used to reference this point in the definition of
+surfaces or polylines.

Documentation/ProjectFile/gml/points/point/a_name.md

-\ogs_missing_documentation
+Name of the point. It is used to reference the point in the project file.

Documentation/ProjectFile/gml/points/point/a_x.md

-\ogs_missing_documentation
+x-coordinate of the point.

Documentation/ProjectFile/gml/points/point/a_y.md

-\ogs_missing_documentation
+y-coordinate of the point.

Documentation/ProjectFile/gml/points/point/a_z.md

-\ogs_missing_documentation
+z-coordinate of the point.

Documentation/ProjectFile/gml/points/point/i_point.md

-\ogs_missing_documentation
+Defines a single point.

Documentation/ProjectFile/gml/polylines/i_polylines.md

-\ogs_missing_documentation
+Contains polyline definitions.

Documentation/ProjectFile/gml/polylines/polyline/a_id.md

-\ogs_missing_documentation
+ID of the polyline. Currently unused.

Documentation/ProjectFile/gml/polylines/polyline/a_name.md

-\ogs_missing_documentation
+Name of the polyline. Used to reference it in the project file, i.e., for instance to set boundary conditions.

Documentation/ProjectFile/gml/polylines/polyline/i_polyline.md

-\ogs_missing_documentation
+Defines a single polyline.
+A polyline is a sequence of line segments, which is given here by a list of
+points; two adjacent points define one line segment.

Documentation/ProjectFile/gml/polylines/polyline/t_pnt.md

-\ogs_missing_documentation
+References a \ref ogs_file_param__gml__points__point "point".
+The content of this tag is the respective \ref ogs_file_attr__gml__points__point__id "point ID".

Documentation/ProjectFile/gml/surfaces/i_surfaces.md

-\ogs_missing_documentation
+Contains surface definitions.

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

-\ogs_missing_documentation
+ID of the surface. Currently unused.

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

-\ogs_missing_documentation
+The name of the surface. Used to reference it in the project file, i.e., for instance to set boundary conditions.

Documentation/ProjectFile/gml/surfaces/surface/element/a_p1.md

-\ogs_missing_documentation
+The first triangle point.
+This attribute references a \ref ogs_file_attr__gml__points__point__id "point ID".

Documentation/ProjectFile/gml/surfaces/surface/element/a_p2.md

-\ogs_missing_documentation
+The second triangle point.
+This attribute references a \ref ogs_file_attr__gml__points__point__id "point ID".

Documentation/ProjectFile/gml/surfaces/surface/element/a_p3.md

-\ogs_missing_documentation
+The third triangle point.
+This attribute references a \ref ogs_file_attr__gml__points__point__id "point ID".

Documentation/ProjectFile/gml/surfaces/surface/element/i_element.md

-\ogs_missing_documentation
+Defines a triangle by its three points.
+The face normal of the triangle is determined such that the points are
+located counter-clockwise around it (right-hand rule).