Continuing tweaking search relevance

[Mpdreamz]

Summary

• Add rank_feature scoring for navigation structure - Landing/overview pages now score better based on their navigation depth and position. navigation_depth and
  navigation_table_of_contents fields are indexed as rank_feature with negative impact (shallower = better). Reference and getting-started sections get slight boosts.
• Improve synonym handling - Split synonyms into index-time and search-time. Key terms like esql, data-streams, and machine-learning now use explicit contraction rules (e.g.,
  es|ql => esql, data stream => data-streams) applied at index time for consistent matching.
• Tune lexical query scoring - Wrap title completion match in ConstantScoreQuery to prevent high TF/IDF from dominating. Reduce body match boost. Add phrase matching for 3+
  token queries. Remove redundant title tokens from search_title to avoid inflating TF.

Test plan

• Run existing search relevance tests - 14 new test cases added covering single-term product searches (elasticsearch, kibana, logstash, etc.), synonyms (ml, esql), and longer
  queries
• Verify landing pages like /docs/reference/elasticsearch rank first for "elasticsearch"
• Verify synonym searches like "ml" and "machine learning" return same top result
• Confirm deeper nested pages don't outrank overview pages for generic terms

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This is not complete but ready to be merged, this is a another step closer to increase relevance in continutation of #2279

Not all relevance test pass yet:

q=datastreams

Expected: /docs/manage-data/data-store/data-streams
  - Score: 7.5496
  - Matched: True

Actual: /docs/manage-data/lifecycle/data-stream
  - Score: 7.5838
  - Matched: True

It's close but the top result is not quite what I want.

q=logstash

Expected: /docs/reference/logstash
  - Score: 7.8765
  - Matched: True

Actual: /docs/release-notes/logstash
  - Score: 7.9206
  - Matched: True

Similar its close but because logstash reference is nested deeper than its release-notes its not at in the number 1 spot.

Will continue to follow up with this.