Add 1D cooling Thermo-Mechanical tutorial to documentation

[npillardou]

This PR adds a new validation example under advancedExamples/validationStudies/thermoPoromechanics/
that isolates a single mechanism: cooling a rock that is not free to contract generates
tensile stress, and the constitutive model decides whether that stress leads to failure.

The example reuses the 1DCooling input files already in develop and contrasts two runs
that differ only by the solid model:

• ThermoElastic_1DCooling_fim_smoke.xml — ElasticIsotropic: the induced stress grows
  linearly with cooling, without bound.
• ThermoDruckerPrager_1DCooling_fim_smoke.xml — DruckerPrager: the stress saturates on
  the yield surface and the rock deforms plastically.

Documentation infrastructure

Follows the convention introduced in #3884: the .hdf5 outputs are not versioned.

• postprocess1DCooling.py — run locally, reads the TimeHistory outputs and writes a single
  20 kB CSV.
• plot1DCooling.py — run by Sphinx at build time, reads that CSV only. Depends on numpy
  and matplotlib alone, no h5py and no mpmath.

Files

file	change
1DCooling/Example.rst	new tutorial page
1DCooling/AnalyticalSol.py	reference analytical solution
1DCooling/postprocess1DCooling.py	new, local post-processing
1DCooling/plot1DCooling.py	new, build-time figure
1DCooling/cooling1D.csv	new, plotted data (20 kB)
thermoPoromechanics/Index.rst	toctree entry
3 × inputFiles/thermoPoromechanics/*1DCooling*.xml	SPHINX comment markers only

Notes for reviewers

• The three input files are touched only to add <!-- SPHINX_... --> comment markers, so
  the tutorial can literalinclude the constitutive and boundary-condition blocks directly
  from the XML rather than duplicating them. No physics, mesh, solver or event is changed.
• No new integrated test: the three 1DCooling decks are already declared in
  thermoPoromechanics.ats on develop.
• To regenerate the CSV after a change to the cases, run both decks in their own directories
  and call postprocess1DCooling.py -e <elastic_dir> -d <druckerPrager_dir>.

[frankfeifan]

Thanks!