[Doc] added input file documentation

Documentation/ProjectFile/prj/process_variables/process_variable/boundary_conditions/boundary_condition/Python/c_Python.md

+Boundary condition using a Python script to compute its values.
+
+See ProcessLib::PythonBoundaryConditionPythonSideInterface for the Python-side
+BC interface.

Documentation/ProjectFile/prj/process_variables/process_variable/boundary_conditions/boundary_condition/Python/t_bc_object.md

+The object (variable name) in the provided Python script that will be used for
+this BC.

Documentation/ProjectFile/prj/process_variables/process_variable/boundary_conditions/boundary_condition/Python/t_flush_stdout.md

+Determines if stdout should be flushed each time before and after passing control to the Python script.
+This might slow down operation, but ensures the right order of messages printed
+to stdout from Python and C++.

Documentation/ProjectFile/prj/t_python_script.md

+Path to a Python script file (absolute or relative to the prj file) that is
+executed by OpenGeoSys if OpenGeoSys has been built with Python support.
+
+This script can be used, e.g., to define boundary conditions that can be
+referenced from the prj file and used by OpenGeoSys.