[Docs] Document video caption-quality evaluation with Summarize-then-Align #2125

Context

Child of #2118. #1980 added eval/video/ tooling for caption-quality evaluation with CosmosEmbed1. Documentation exists only in eval/video/README.md, outside the published Fern site.

Scope

• Benchmark-dataset construction: sampling, embedding, K-means selection, and expected artifacts
• Caption scoring: summarization, CosmosEmbed1 encoding, cosine similarity, caching, and CSV outputs
• Model/data prerequisites and expected directory layouts
• Interpretation and limitations of the baseline scores

Acceptance criteria

• A published Fern evaluation guide is added and linked from video captioning docs
• Both scripts have tested commands and complete argument explanations
• Input/output schemas and directory layouts are shown
• Hardware/model-download expectations and reproducibility limits are explicit
• The distinction between evaluation tooling and the main curation pipeline is clear
• Fern checks pass

Related PR: #1980