Ref guide - Improve docs for Dense Vector reranking examples

[adamjq]

Description

The Dense Vector Search page in the Solr Ref Guide currently has a section titled Usage in Re-Ranking Query. However, it only contains a single example of re-ranking with the KNN query parser which has an important caveat that this initiates a second pass full KNN query. While this is true, I think it may be confusing to readers as it's not the only supported option for re-ranking documents with vectors.

This PR adds documentation for the more efficient re-ranking feature that rerank subsets of documents by direct vector comparisons with the vectorSimilarity Function, with some notes about motivations and tradeoffs.

Solution

• Moves indentation level of Usage in Re-Ranking Query in Dense Vector Search ref guide
• Adds examples of how to use {!func}vectorSimilarity in a re-rank query
• Adds note about reranking with Dense Vector Fields in Query Re-Ranking page

Before

After

Checklist

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• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
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