Feature: add kapa.ai Ask AI assistant to the site

[tuhaihe]

Embed the kapa.ai website widget via the Docusaurus scripts option, adding an "Ask AI" button on every page that answers questions from the Apache Cloudberry documentation.

To follow the ASF privacy policy, the widget sets no tracking cookie, uses hCaptcha instead of Google reCAPTCHA for bot protection, and shows a consent screen before the first question is sent, similar to what the Apache Doris website does.

See: Issue#401 #401

Assisted-by: Claude Code

closes: #ISSUE_Number

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[tuhaihe]

Screenshot:

[leborchuk]

Cannot check it locally, but all looks great, let's see how it works on out site