[GeoMechanicsApplication] Replace two images with matrix equations by proper math blocks

[avdg81]

📝 Description

We have found that using pairs of $$ to enclose a math block in GitHub Markdown is not as robust as it should be. As a result, when the math rendering failed we created images of the equations to be displayed. After some searching I've found that there is an alternative (recommended) way of wrapping mathematical equations in GitHub Markdown (rather than using pairs of $$):

```math
(...the math goes here...)
```

When I wrapped the original LaTeX code for initial effective stresses using POP in such a block, the rendering worked as expected. This PR replaces two images of matrix equations by code that automatically generates these images.

Note that the above syntax is also documented here on GitHub Docs.

[WPK4FEM]

Short and clear. Let's find the other occurrences in another PR.

[indigocoral]

Thank you for creating this pull request and finding a fix for it!