Merge branch 'CleanupTMDocu' into 'master'

[doc/TM] Drop unused tags documentation.

See merge request ogs/ogs!3457

Documentation/ProjectFile/prj/processes/process/THERMO_MECHANICS/t_linear_thermal_expansion_coefficient.md

-Linear expansion changes in one dimension.
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Documentation/ProjectFile/prj/processes/process/THERMO_MECHANICS/t_reference_solid_density.md

-Reference solid density \f$\rho_{\rm SR0}\f$.
-
-The real solid's density (the density of the solid grains) is expressed as:
-\f[\rho_{\rm SR} = \frac{\rho_{\rm SR0}}{1 + 3 * \alpha_{\rm T} * \Delta T }\f]
-under the assumption of mechanically incompressible solid.
-
-The original Ansatz made is \f[\rho_{\rm SR} = \rho_{\rm SR0} {\rm exp} (-3
-\alpha_{\rm T} \Delta T)\f] for large thermal strains. Once we linearize around
-\f$\Delta T = 0\f$ and truncate the Taylor series after the first member, the
-end result is the above formula \f$\rho_{\rm SR} = \rho_{\rm SR0} / ( 1 + 3
-\alpha_{\rm T} \Delta T)\f$.  One can also arrive at the result by assuming a
-linear volume strain from the start: \f$e = 3 \alpha_{\rm T} \Delta T\f$. The
-key assumption is that the solid density is independent of pressure, _i.e._:
-\f$\rho_{\rm SR} = \rho_{\rm SR}(T)\f$.

Documentation/ProjectFile/prj/processes/process/THERMO_MECHANICS/t_specific_heat_capacity.md

-Specific heat capacity is the heat capacity per unit mass of a material.
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Documentation/ProjectFile/prj/processes/process/THERMO_MECHANICS/t_thermal_conductivity.md

-Thermal conductivity is the property of a material to conduct heat.
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