[Docs] Dem Sampling Docs, follow-up #479

[kvmto]

This is a follow-up for #479

Summary

Adds comprehensive documentation for the dem_sampling feature across the Sphinx doc tree: API references (Python and C++), a dedicated example page with narrative explanation, and a new section in the QEC introduction page.

Changes

• docs/sphinx/examples_rst/qec/dem_sampling.rst — New dedicated page covering the DEM sampling workflow, GPU acceleration pipeline (sparse Bernoulli sampling, GF(2) sparse-dense multiply, bitpacking), input type handling (NumPy, PyTorch CUDA, PyTorch CPU), and backend selection logic.
• docs/sphinx/examples/qec/python/dem_sampling.py — Runnable Python example: basic usage, syndrome verification, reproducibility check, explicit CPU backend.
• docs/sphinx/examples/qec/cpp/dem_sampling.cpp — Runnable C++ example using dem_sampler::cpu::sample_dem with syndrome verification.
• docs/sphinx/components/qec/introduction.rst — Added "DEM Sampling" subsection under "Numerical Experiments" with inline Python/C++ snippets and a link to the full example page.
• docs/sphinx/api/qec/python_api.rst — Added autofunction:: cudaq_qec.dem_sampling section.
• docs/sphinx/api/qec/cpp_api.rst — Added doxygenfunction entries for all 5 C++ API functions (cpu::sample_dem x2, gpu::sample_dem, legacy dem_sampling x2).
• docs/sphinx/examples_rst/qec/examples.rst — Added dem_sampling.rst to the examples toctree.

Testing

• All 4 code snippets (Python standalone, Python inline, C++ standalone, bash compile command) compiled/ran successfully.
• Sphinx build (ninja docs) succeeds with no new warnings.
• Rendered HTML verified for correct heading hierarchy, content, and cross-links.