docs(examples): add elastic footing geomechanics example

[kilickursat]

Summary

This PR adds a small elastic strip-footing geomechanics boundary-value example
under examples/solid/geomechanics/.

Unlike the existing Drucker-Prager triaxial example, which is a constitutive
driver, this example solves a real linear-elastic boundary-value problem:
a soil block with roller-style supports is loaded by a centered footing patch,
then solved with TensorMesh's existing direct linear-elasticity workflow.

Scope

• Adds examples/solid/geomechanics/elastic_footing/
• Uses the existing LinearElasticityElementAssembler
• Uses Condenser for vector-valued displacement boundary conditions
• Applies a lumped top-surface footing load
• Adds a settlement/deformed-mesh visualization
• Adds a docs page and gallery card thumbnail
• Adds CPU + float64 tests for load balance and linear load scaling
• No public API changes
• No core TensorMesh code changes

Validation

Ran locally:

• python examples/solid/geomechanics/elastic_footing/elastic_footing.py --no-plot --chara-length 0.55
• pytest tests/assemble/test_elastic_footing_example.py -v
• pytest tests/assemble/test_drucker_prager_triaxial_example.py -v
• cd docs && make html
• cd docs && make zh

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

Notes

The footing pressure is lumped over top-surface nodes for a compact educational
example, following the style of the existing direct linear-elastic solid examples.
The reaction/load-balance test verifies that the total applied load is recovered
by the constrained vertical DOFs.

This PR intentionally does not add tensormesh/assemble/geomechanics.py or a
public material API. It is a small example-only step toward a geomechanics
boundary-value example family.