docs: update MNNVL user guide for single-annotation model

[shmuel-runai]

/kind documentation
Ref #417

Summary

• Rewrites docs/user-guide/auto-mnnvl.md to reflect the new single-annotation model (grove.io/mnnvl-group) introduced in refactor(mnnvl): replace two-annotation model with single mnnvl-group annotation #574, replacing the previous two-annotation system (grove.io/auto-mnnvl + grove.io/mnnvl-group).
• Adds usage examples covering simple opt-in, multiple groups, PCS-level opt-out, and a full three-layer hierarchy (PCS → PCSG → PCLQ) with overrides.
• Documents annotation propagation, ComputeDomain naming, group name validation, backward compatibility (breaking change from old annotation), and updated limitations.

Test plan

• Review rendered markdown for correctness and readability
• Verify YAML examples match current API and annotation semantics
• Cross-check with GREP-417 and merged implementation (refactor(mnnvl): replace two-annotation model with single mnnvl-group annotation #574)

[copy-pr-bot]

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