Relabel middleware docs

[danielgerlag]

This pull request adds comprehensive documentation for the new relabel middleware, which enables label normalization and remapping for nodes and relationships in source data. The documentation covers configuration, usage examples, and important behavioral details to help users understand how to apply label mappings to streamline and standardize their data pipelines.

Middleware documentation improvements

• Added a new section describing the relabel middleware, including its purpose, configuration options, and behavioral details for processing SourceChange events.

Usage examples and guidance

• Provided a basic example demonstrating how to normalize personnel-related labels for querying User entities.
• Included a table showing before-and-after results of label remapping, clarifying how unmapped labels are retained.
• Added an example for relationship label remapping, illustrating how relation labels like KNOWS can be rewritten to domain-specific terms such as CONNECTED_TO.
• Explained that relabeling applies to Insert, Update, and Delete events, ensuring consistency of mapped labels throughout the data lifecycle.