[Docs] Add a FAQ page

### Search before asking

- [X] I searched in the [issues](https://github.com/alibaba/fluss/issues) and found nothing similar.

### Motivation

Currently, there are many users have interests in Fluss, and there are many common questions, such as "what's the difference between Fluss and Kafka?", "what's the difference between Fluss and Paimon?". We can create a FAQ page to address such questions.

### Solution

Maybe we can add a "Project" dropdown section in the top menu, and move "Community", "Roadmap" under the "Project" dropdown menu. Add the "FAQ" under "Project" as well.

### Anything else?

_No response_

### Willingness to contribute

- [ ] I'm willing to submit a PR!

Start by locating the documentation site navigation and the existing Community and Roadmap pages mentioned in the issue. Review how project pages are organized, then add an FAQ page covering common Fluss questions and place it under a new Project menu; done means the page is reachable and the navigation remains usable.

Written by the indexing model from the issue text.