[Doc] added nonlinear solver documentation

Documentation/ProjectFile/prj/nonlinear_solvers/i_nonlinear_solvers.md

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+Contains nonlinear solver settings.

Documentation/ProjectFile/prj/nonlinear_solvers/nonlinear_solver/Picard/c_Picard.md

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Documentation/ProjectFile/prj/nonlinear_solvers/nonlinear_solver/i_nonlinear_solver.md

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+Defines a single nonlinear solver.

Documentation/ProjectFile/prj/nonlinear_solvers/nonlinear_solver/t_linear_solver.md

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+Specifies the linear solver used to solve the linearized equation systems.
+
+The linear solver is referenced by \ref ogs_file_param__linear_solver__name
+"name".

Documentation/ProjectFile/prj/nonlinear_solvers/nonlinear_solver/t_max_iter.md

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+Specifies the maximum number of iterations the nonlinear solver can make for
+solving a single equation system.
+
+If this number is exceeded, the nonlinear solver failed.

Documentation/ProjectFile/prj/nonlinear_solvers/nonlinear_solver/t_name.md

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+Name of the nonlinear solver.
+
+Used to specify which process is solved by which nonlinear solver.

Documentation/ProjectFile/prj/nonlinear_solvers/nonlinear_solver/t_type.md

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+Type of the nonlinear solver.
+
+Can be <tt>Picard</tt> or <tt>Newton</tt>.