Add documentation for FIP (Flink Improvement Proposal) contribution guide

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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!