Update doxygen for LIE fracture models.

There was an error in CI (which is already fixed) for parsing the doxygen output and so the missing/incorrect documentation slipped in.

The LIE fracture model are inconsistent; the linear elastic model is (mis)using the solid's linear elastic material, Mohr-Coulomb has its own documentation/folder but also inside the solid material models.

In this PR:

• Moving all fracture models documentation in own folder corresponding to the fracture_model tag.
• Filling out all of the missing documentation.
• Updating the project files for the new prj file structure.