Document differentiable GVEC

[krystophny]

Documents the differentiable GVEC contract.

• New docs/user/differentiable-gvec.md: build options, derivative levels with the raw-residual sign identity, a minimal Python example, the external-optimizer and adjoint examples, the SIMSOPT boundary-variable rule for LA coefficients, and the support matrix (h-maps, OpenMP, MPI, no GPU, no free boundary).
• Linked from the user-guide toctree and from other-interfaces.md.

Closes #28.

Verification

Before

$ test -f docs/user/differentiable-gvec.md; echo $?
1

After

$ test -f docs/user/differentiable-gvec.md; echo $?
0
$ grep -c differentiable-gvec docs/user/index.md docs/user/other-interfaces.md
docs/user/index.md:1
docs/user/other-interfaces.md:1

Writing checker passed at the soft threshold.