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docs(guides): add a getting-started guide for SKQD Krylov circuits (#217)

* docs(guides): add a getting-started guide for SKQD Krylov circuits

Add a getting-started guide that walks through building the quantum circuits
for Sample-based Krylov Quantum Diagonalization (SKQD, arXiv:2501.09702), using
the single-impurity Anderson model as the running example. It covers the full
path from a fermionic Hamiltonian to a ground-state energy, entirely in terms
of this package's builtin gates and transpilation rather than hand-assembled
gate networks.

The guide is organized around one idea: SKQD only works when the ground state
is sparse in the computational basis, so that a classical diagonalization can
reconstruct it from a manageable number of sampled bitstrings. For this model
that sparsity is not automatic -- it is a property of the single-particle basis.
The guide makes this concrete by rotating the Hamiltonian from the position
basis into the momentum (diagonal-bath) basis and counting the determinants
needed for 99% of the ground-state weight in each: thousands in position, a
couple of dozen in momentum, at identical energy. Everything downstream is then
built in the momentum basis.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

* Minor improvements after review

* docs: reduce logging verbosity during build

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Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

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