Docs: Add how-to guide for panel plugin transformations

[leventebalogh]

Fixes https://github.com/grafana/grafana-catalog-team/issues/279

What changed?

Added a new how-to guide at docs/how-to-guides/panel-plugins/work-with-transformations.md that explains how data transformations affect panel plugins. It covers the data pipeline (query → transformations → field overrides → panel), defensive data validation, letting users choose fields, handling multiple data frames, and using @grafana/data utility functions like getFieldDisplayValues.

The guide is developer-focused and links to the existing Grafana user docs for transformation configuration details.

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[leventebalogh]

Closing in favour of #2756.