[GeoMechanicsApplication] Add documentation for hardening in Mohr-Coulomb in the form of markdown

[mnabideltares]

📝 Description
Add documentation that explains the purpose, setup, and expected results of the unit tests for the Mohr–Coulomb hardening/softening implementation. This ensures reproducibility, helps new developers understand the test suite, and makes it easier to verify correctness.

🆕 Changelog

• Extended Readme.md

[WPK4FEM]

Dear Mohamed,
Thank you for the large effort of putting this documentation properly in markdown. Still I have quite some remarks. Adding the picture of the zones that we have used before would be very helpful.
Regards, Wijtze Pieter

[WPK4FEM]

A flow diagram is probably faster to understand than writing this in words.

[WPK4FEM]

Thank you for the diagram. I have a couple of remarks about it:
1 using a bent curve look not common to me, please use straight lines and corners.
2 the shape that encompasses "Map trial stresses" is used for choices. Around "Map trial stresses" I would expect a rectangular box.
3 the shape that encompasses "Map trial stresses" I do expect around the convergence criterion that decides whether you are ready or should loop.

Further:

• the lambda probably is lambda dot
• "update plastic strain" the action is more "calculate plastic strain increment"
• in the preceding text we should mention that kappa is the state variable. in this case its content is an equivalent plastic strain. That is a more commonly used description than hardening variable.
• material parameters are phi_0, psi_0 and c_0, H_phi, H_psi and H_c. Here the state variable dependent parametersare updated.

[WPK4FEM]

Hi Mohamed,
Thank you for the work, writing documentation is a tedious thing. In the issue, it also mentions documentation for unit tests. That I would remove, the unit tests should be self-documenting. The work here documents the implementation.

Reading again I realize that previous time I should have opened another view. Looking at just the rendering gave a way easier readable document. Now I have a lot more comments, but I hope they are easy to answer.

Regards, Wijtze Pieter