[GeoMechanicsApplication] Add documentation for hardening in Mohr-Coulomb

[mnabideltares]

Title: Add documentation for Mohr-Coulomb hardening/softening tests to the ReadMe file

Summary

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.

Requirements / acceptance criteria

• For each test, document:
  • Purpose: what aspect of the model it validates (elastic limit, plastic onset, hardening, softening, tangent, etc.).
  • Setup: initial parameters, loading path, strain increments.
  • Expected result: qualitative description (e.g. stress remains elastic, cohesion increases, yield ~ 0).
  • Pass criteria: tolerance or numerical check used in the code.
• Add references to relevant sections of theory/user docs if available.