[Doc] added docu for the Parameter class and its implementations

Documentation/ProjectFile/parameter/MeshElement/c_MeshElement.md

+Defines a parameter which is piecewise constant on each mesh element.

Documentation/ProjectFile/parameter/MeshElement/t_field_name.md

+The name of the mesh property that contains the parameter values.

Documentation/ProjectFile/parameter/MeshNode/c_MeshNode.md

+Defines a parameter with values in each mesh node.

Documentation/ProjectFile/parameter/MeshNode/t_field_name.md

+The name of the mesh property that contains the parameter values.

Documentation/ProjectFile/parameter/i_parameter.md

+Defines a parameter.
+
+A Parameter is a scalar- or vector-valued field in space and time.

Documentation/ProjectFile/parameter/t_name.md

+Sets the name of the parameter.
+
+The parameter can be referenced by this name at the places where it is used,
+e.g., in initial and boundary conditions, or in the local assembler.

Documentation/ProjectFile/parameter/t_type.md

+Sets the parameter type.

Documentation/ProjectFile/prj/parameters/i_parameters.md

+Contains parameter definitions.

Documentation/ProjectFile/prj/parameters/t_parameter.md

+Defines a single parameter.
+
+See \ref ogs_file_param__parameter.