[DOC] Guide: Add your own metric

[ividal]

If a new contributor wants to add a new metric (either already available through the integrated libraries or only through a new one), what would they have to do?

Scope:

• Metric applies to existing use cases (a separate guide would cover how to add a whole new use case)
• User wants to enable a new metric at API level (i.e. UI is out of scope for this guide)
• Case where a GPU is not needed
• Case where a GPU is recommended for the new metric (e.g. llm as a judge or embeddings-based metrics)