[Feature] Add Kapa.ai docs assistant integration to DolphinScheduler

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/dolphinscheduler/issues?q=is%3Aissue) and found no similar issues.

### What happened

We already applied for and got access to Kapa.ai for the DolphinScheduler project. I’d like to propose that we integrate the Kapa.ai documentation assistant into our docs of [https://github.com/apache/dolphinscheduler-website](https://github.com/apache/dolphinscheduler-website)

Previously, DolphinScheduler had a corresponding PR. However, at that time, the act of connecting to the AI Q&A was not approved by ASF, so the PR was rolled back. You can refer to that PR.

**Why**

* DolphinScheduler has a lot of concepts and configuration options.
* New users often ask repeated “how to configure X / why does Y fail” questions.
* A docs-grounded AI assistant can help users find answers faster and reduce basic Q&A.

**What needs to be done**

**Docs site integration**

* Embed the Kapa “Ask AI” widget `` snippet into the official documentation site layout, so users can ask questions on any docs page.

### Are you willing to submit PR?
- [ ] Yes I am willing to submit a PR!

### Code of Conduct

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