Update documentation for test setup simplification and Coefficient VTK post-processing

[CINTROINI]

Summary

Updates the documentation to reflect the recent SLOTH additions: simplified test
setup aliases/factories (using namespace SlothND, set(Periodic)SpatialDiscretization,
setBoundaryConditions, setCoupling<N>), auxiliary variables set after Problem
construction, and Coefficient VTK post-processing.

New page

• Documentation/User/Aliases/index.md — new reference page for using namespace SlothND:
  what's available from Sloth1D/Sloth2D/Sloth3D alone (FECollection, VARS, VAR,
  PST, SPA, SPAS, BCS, PB_MPI, PB_CALPHAD, PB_PROPERTY), the PDE aliases
  (TransientOPE/TransientPB/SteadyOPE/SteadyPB, already included, no extra namespace
  needed), and a practical cheat sheet for the factory functions.

Updated pages

• SpatialDiscretization/Meshing/index.md — documents set(Periodic)SpatialDiscretization
  for building N spatial discretizations sharing a mesh.
• SpatialDiscretization/BoundaryConditions/index.md — documents setBoundaryConditions;
  also documents space-/time-variable Dirichlet boundary conditions via a Glossary::Dirichlet
Coefficient (previously undocumented, already supported in code).
• MultiPhysicsCouplingScheme/Couplings/index.md — documents setCoupling<N> for coupling
  N identical Problems.
• MultiPhysicsCouplingScheme/Problems/PDEs/index.md — documents set_auxvariables (set
  after construction instead of through the constructor) and set_vtk_coefficients.
• PostProcessing/index.md — new "Post-processing Coefficients in VTK output" section.
• Started/HowTo/Simple/index.md — points to the new Aliases page;