docs: make the landing page show how the package is used

[Sophia Wen (hfwen0502)]

The documentation home page was a few sentences pointing elsewhere. This makes it show how the package is used, so a reader can tell from the landing page alone what SBD does here and where to go next:

• Installation: the common CPU case as a copyable conda + pip recipe (no wheels are published, so the extension compiles locally), with GPU backends, host-MPI builds and every build option left to INSTALL.md.
• SBD as the solver of an SQD loop: a complete solve_sci_batch example plugged into qiskit-addon-sqd's diagonalize_fermionic_hamiltonian, plus a sentence on SBD-selected carryover (carryover_type, qiskit-addon-sqd >= 0.14.0).
• Backends: what cpu, gpu and gpu-omp run on and what builds each. gpu-omp is marked as offloading TPB only: SBD has no OpenMP-offload GDB code, so a GDB run under gpu-omp executes on the host, and GDB on GPUs needs gpu (Thrust). On 8x H100, GDB on 2.38M determinants took 16 s on gpu against 101 s on gpu-omp, which matched cpu at 94 s.
• TPB or GDB: the two ways of spanning the subspace, and that only TPB is part of the SQD-loop integration.
• More: links to the README, the examples, the API reference and issues.

Only docs/index.rst changes. sphinx-build -W -T --keep-going, the command tox -e docs runs, succeeds with no warnings.

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