[FEATURE] Add Flink lineage support documentation

### Describe the feature

Gravitino has started preparing support for Flink lineage, add user-facing documentation for Gravitino Flink lineage support. The OpenLineage Flink plugin enables table-level lineage collection for Flink SQL and DataStream API jobs, transforming dataset identifiers to Gravitino format (namespace=metalake, name=catalog.db.table).

### Motivation

Gravitino already has Spark lineage documentation (`gravitino-spark-lineage.md`), but lacks equivalent documentation for Flink lineage. Users need a reference guide for configuring and using the Flink lineage plugin.

### Describe the solution

Add `gravitino-flink-lineage.md` covering capabilities, configuration, connector support matrix, and usage examples.Update `lineage.md` to reference the new Flink lineage page.

### Additional context

_No response_

Read gravitino-spark-lineage.md first for the existing lineage documentation structure, then review lineage.md to see where the new page should be referenced. Add gravitino-flink-lineage.md covering capabilities, configuration, the connector support matrix, and usage examples; update lineage.md so users can find it.

Written by the indexing model from the issue text.