docs: README covers the UMAT layer, plasticity and every registered type (#72)

[petlenz]

Closes #72.

The README had zero mentions of UMAT, CalculiX, Abaqus, plasticity or docs/, and listed 13 of the 29 registered types. Everything a new user would come for -- the return maps, the hardening laws, the solvers, the UMAT layer, the CalculiX hook, both theory documents -- was invisible from the front page.

Adds:

• a documentation index pointing at docs/plasticity.md and docs/plasticity-theory.md;
• a plasticity section: the split between return map, yield surface and hardening law, and what each registered type is;
• a UMAT section with both entry-point macros (NUMSIM_MATERIALS_DEFINE_UMAT and NUMSIM_MATERIALS_DEFINE_CALCULIX_BEHAVIOUR), noting that the CalculiX hook takes PK2 and dS/dE, and that small-strain materials are valid for large rotation with small strain only;
• the complete type list, all 29, grouped by area;
• install-and-consume instructions;
• a note that commit()/revert() belong to the driver.

The opening example is a J2 deck with every parameter name checked against the materials' own schemas (local_newton takes tolerance, not tol; sigma_0 is on j2_plasticity, not on the hardening material). It also flags the K collision -- bulk modulus on one material, hardening modulus on another -- which #73 tracks.

rk_integrator is deliberately absent from the registered-type table: it is not registered, which is #61.