Add documentation for FIP (Flink Improvement Proposal) contribution guide

### Search before asking

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

### Description

## Summary
The Fluss project currently lacks a formalized process and clear guidelines for submitting and reviewing Fluss Improvement Proposals. New contributors and community members often struggle to understand the proposal lifecycle, required format, and review criteria. This issue tracks the creation of comprehensive documentation to streamline FIP contributions.

### Willingness to contribute

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

Start by reviewing the repository’s existing documentation structure and any current references to Fluss Improvement Proposals. Define the proposal lifecycle, required format, submission process, and review criteria in a contributor-facing guide. Done means a newcomer can understand how to prepare and submit a FIP and how review proceeds.

Written by the indexing model from the issue text.