feat(index): support distributed IVF_RQ vector indexes

[hfutatzhanghb]

What changed

• Add IVF_RQ to the distributed vector index type allowlist.
• Keep IVF_RQ on the IVF-only training path, without PQ codebook training.
• Document IVF_RQ in the distributed indexing guide.

Testing

• python3.10 -m pytest --noconftest -o addopts='' tests/test_vector_index_options.py -q

[chenghao-guo]

Thanks a lot for your contribution!