docs(examples): show Drucker-Prager triaxial response

[kilickursat]

Summary

This PR adds the generated Drucker-Prager triaxial compression response figure to the already-merged geomechanics example page and uses the same image as the Solid Mechanics gallery card thumbnail.

The example already writes drucker_prager_triaxial.png during a normal run. This PR follows the existing solid-mechanics docs pattern by copying that output into docs/source/_static/solid_mechanics/ and embedding it with a .. figure:: directive.

Scope

• Adds docs/source/_static/solid_mechanics/drucker_prager_triaxial.png
• Embeds the figure in docs/source/example_gallery/solid/drucker_prager_triaxial.rst
• Adds the image as the gallery card thumbnail in docs/source/example_gallery/solid/index.rst
• No public API changes
• No core code changes

Validation

Ran locally:

• python examples/solid/geomechanics/drucker_prager_triaxial/drucker_prager_triaxial.py --no-plot --steps 16
• pytest tests/assemble/test_drucker_prager_triaxial_example.py -v
• cd docs && make html

Checked that the generated docs page renders the new figure and that the Solid Mechanics gallery card has a thumbnail.