[DOCS] Enhance ConfigOptionsDocGenerator for cleaner MDX output and fixed formatting

### Search before asking

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

### Description

Description
Task: Improve the ConfigOptionsDocGenerator to ensure the auto-generated configuration documentation is professional, accurate, and free of formatting artifacts.

Purpose: The current documentation generator had several minor issues that impacted the readability of the official docs.

### Willingness to contribute

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

Start by locating ConfigOptionsDocGenerator and inspect how it produces the generated configuration documentation. Compare its current MDX output with the official docs to identify formatting artifacts and accuracy issues. Done means the generated documentation is professional, accurate, and free of the reported formatting problems, with the expected output verified.

Written by the indexing model from the issue text.