Add Grafana related content in contributing guideline

[joshuayao]

OPEA will introduce OpenTelemetry Tracing for GenAI examples since v1.3. This update includes the addition of Grafana dashboards to visualize tracing data. To ensure clarity and consistency, we need to provide guidelines on where to place the Grafana configuration files.

The recommended location for these files is:
GenAIExamples/<EXAMPLE_NAME>/benchmark/grafana/

Please contact @louie-tsai, the contributor of OpenTelemery tracing, for more details.

The owner is @rbrugaro

[yinghu5]

Hi @louie-tsai @rbrugaro just remind ,the issue is open, please help to update if any progress