chore(docs): enhance documentation for prompt evaluation

Alexander Chueshevrequested to merge
ac/prompt-evals-docs into main

What does this merge request do and why?

Updates documentation to better present prompt evaluations withing AIGW following gitlab-org/modelops/ai-model-validation-and-research/ai-evaluation/prompt-library#721 (closed).

Merge request checklist

  • Tests added for new functionality. If not, please raise an issue to follow up.
  • Documentation added/updated, if needed.

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