[Doc] documented curves

Documentation/ProjectFile/prj/curves/curve/i_curve.md

+Defines a single curve.
+
+The curve is given by a set of coordinates (the arguments) and a set of values
+at these coordinates. The values for any arguments between the given coordinates
+are obtained via linear interpolation.

Documentation/ProjectFile/prj/curves/curve/t_coords.md

+Coordinates at which the curve's values are given.

Documentation/ProjectFile/prj/curves/curve/t_name.md

-\ogs_missing_documentation
+Name of the curve.
+
+It is used to reference the curve, e.g., in parameter definitions.

Documentation/ProjectFile/prj/curves/curve/t_values.md

+Values of the curve at the given coordinates.

Documentation/ProjectFile/prj/curves/i_curves.md

+Contains curve definitions.
+
+Curves are scalar valued functions of a scalar argument.
+They can be used, e.g., to define time-dependent quantities.