[DOC][AI Assistant] Add documentation for AI Assistant

[xluo-aws]

What do you want to do?

• [ ] Request a change to existing documentation
• [X] Add new documentation
• [ ] Report a technical problem with the documentation
• [ ] Other

Tell us about your request. Introduce AI Assistant features

*Version: 2.17.

What other resources are available?

[xluo-aws]

Could you update tag to 2.18? Our change is not merged in time for 2.17

[xluo-aws]

Please assign to @ruanyl

[xluo-aws]

Here is one of the PRs https://github.com/opensearch-project/documentation-website/pull/8622