Document supported and unsuppored SVS variants

Currently, the SVS variant on PyPI is supported and automatically installed when the installation steps are followed. The SVS variant at https://github.com/intel/ScalableVectorSearch is not supported and should be removed before installing svsbench. This should be documented.

Review the installation steps and the supported PyPI SVS variant, then compare them with the unsupported variant at https://github.com/intel/ScalableVectorSearch. Update the documentation to identify the supported and unsupported variants and state that the unsupported one should be removed before installing svsbench.

索引モデルが issue の本文から書いたものです。