Align docs with pluggable benchmark API

[jnke2016]

Description of changes

cuVS Bench documentation updates to align with the pluggable benchmark API:

• index.rst: Fix datasets.yaml path and document the constraints field in algorithm YAML; add Recommended (orchestrator API) and Legacy CLI workflow; fix intro typo.
• param_tuning.rst: Add Benchmark modes (sweep vs tune) and tune-mode example.
• pluggable_backend.rst (new): Document config loader + backend pair, run flow, what the loader produces, steps to add a backend, and a minimal Elasticsearch-style example. Add to cuvs_bench toctree.