feat(docs): integrate Kapa.ai Ask AI widget into layout

[KaiSong-UK]

Integrate Kapa.ai documentation assistant into the DolphinScheduler website, allowing users to ask AI-powered questions about DolphinScheduler documentation directly from any docs page.

Changes Made

• Added Kapa.ai widget script to the main layout component (src/views/Layout/index.jsx)
• Configured widget with project-specific settings (website-id, project name, color, button text)
• Ensures script is only injected once and properly cleaned up

Testing

• Widget loads correctly on all documentation pages
• Mobile and desktop responsiveness
• No impact on website performance

Related Issues

• Closes #17775: Add Kapa.ai docs assistant integration to DolphinScheduler

Security Considerations

• Kapa.ai script loaded from official CDN
• No sensitive data exposed to third-party service
• Compliance with ASF policies and guidelines