Add pgvector doc

[xtangcode]

Add pgvector doc to describe how to use pgvector search in cloudberry.

closes: #ISSUE_Number

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[github-actions]

Hiiii, @xtangcode welcome!🎊 Thanks for taking the effort to make our project better! 🙌 Keep making such awesome contributions!

[TomShawn]

Nice job! These are very minor comments on format. PTAL

In addition, to make this new document searchable on the site's left navigation menu, please also update https://github.com/apache/cloudberry-site/blob/main/sidebars.ts#L69 by adding the file name pgvector-search to this line.

[tuhaihe]

Also, can add the doc item to the file sidebars.ts for a better index. Thanks!

[xtangcode]

Also, can add the doc item to the file sidebars.ts for a better index. Thanks!

added.

[TomShawn]

LGTM