feat: optimize elasticsearch ai-search plugin and update related docs"

[cr7258]

Ⅰ. Describe what this PR did

1. linkField 和 titleField 在 Elasticsearch 中改为可选参数。
2. Elasticsearch 搜索的返回字段排除向量内容，避免返回内容过多。
3. 添加 ai-search 集成 Elasticsearch 的教程文档链接。

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.66%. Comparing base (ef31e09) to head (2703d93).
Report is 458 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2100      +/-   ##
==========================================
+ Coverage   35.91%   43.66%   +7.75%     
==========================================
  Files          69       79      +10     
  Lines       11576    12753    +1177     
==========================================
+ Hits         4157     5569    +1412     
+ Misses       7104     6836     -268     
- Partials      315      348      +33     

see 75 files with indirect coverage changes

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[johnlanni]

LGTM