[Doc/InitialStress] Replaced the documentation duplication with symbolic links

...File/prj/processes/process/HYDRO_MECHANICS/initial_stress

...rocesses/process/HYDRO_MECHANICS/initial_stress/a_type.md

...rocess/HYDRO_MECHANICS/initial_stress/i_initial_stress.md

...ion/ProjectFile/prj/processes/process/TH2M/initial_stress

...tFile/prj/processes/process/TH2M/initial_stress/a_type.md

...processes/process/TH2M/initial_stress/i_initial_stress.md

...rocesses/process/THERMO_RICHARDS_MECHANICS/initial_stress

...rocess/THERMO_RICHARDS_MECHANICS/initial_stress/a_type.md

...RMO_RICHARDS_MECHANICS/initial_stress/i_initial_stress.md

...rojectFile/prj/processes/process/initial_stress/a_type.md