[doc] Local coordinate system and parameter tag.

Documentation/ProjectFile/prj/local_coordinate_system/i_local_coordinate_system.md

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+A local coordinate system used for space-dependent tensor transformations.
+
+It offers a simple way for input of anisotropic tensors w.r.t. a coordinate
+system.
+
+The basis vectors form a transformation matrix \f$R = (e_0, e_1, e_2)\f$.
+
+For a given anisotropic tensor \f$A\f$ parameter with the corresponding
+\ref ogs_file_param__prj__parameters__parameter__use_local_coordinate_system
+the tensor is rotated according to the formula: \f$A' = R\cdotA\cdotR^T\f$.
+

Documentation/ProjectFile/prj/parameters/parameter/t_use_local_coordinate_system.md

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+An optional tag indicating whether or not a local coordinate system
+transformation will be applied.
+
+\see \ref ogs_file_param__prj__local_coordinate_system .