[Doc/HM] Copy the documentation of the parameter initial_stress

Documentation/ProjectFile/prj/processes/process/HYDRO_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`.

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

+A 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/HYDRO_MECHANICS/t_initial_stress.md

-\copydoc ProcessLib::HydroMechanics::HydroMechanicsProcessData::initial_stress