[Subtask][#3039] Add SQL REST API docs

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

### Search before asking

- [X] I have searched in the [issues](https://github.com/apache/incubator-kyuubi/issues) and found no similar issues.

### Describe the subtask

Add SQL REST API docs

### Parent issue

https://github.com/apache/incubator-kyuubi/issues/3039

### Are you willing to submit PR?

- [X] Yes. I can submit a PR independently to improve.
- [ ] Yes. I would be willing to submit a PR with guidance from the Kyuubi community to improve.
- [ ] No. I cannot submit a PR at this time.

Read parent issue #3039 first to determine the SQL REST API documentation scope. Then inspect the repository's existing documentation structure and conventions. Done means the requested SQL REST API documentation has been added in the appropriate location and covers the scope defined by the parent issue.

Written by the indexing model from the issue text.