[Doc/TRM] Copied the documentation of the initial stress

Documentation/ProjectFile/prj/processes/process/THERMO_RICHARDS_MECHANICS/initial_stress/a_type.md

+An optional attribute to indicate the type of the initial stress, `total` or
+ `effective`. The default value is `effective`.
+
+\note If subtype = `StressSaturation_StrainPressureTemperature` is bing used
+      in the process setting of `ThermoRichardsMechanics`, this attribute must
+      be presented.

Documentation/ProjectFile/prj/processes/process/THERMO_RICHARDS_MECHANICS/initial_stress/i_initial_stress.md

+An optional tag to input the parameter of initial stress in porous media. It is
+ used in processes `LIE`, `HydroMechanics`, `ThermoHydroMechanics`,
+ `RichardsMechanics`, `ThermoRichardsMechanics`, and `TH2M`.

Documentation/ProjectFile/prj/processes/process/THERMO_RICHARDS_MECHANICS/t_initial_stress.md

-\copydoc ProcessLib::RichardsMechanics::RichardsMechanicsProcessData::initial_stress