Add guidance for writing samples to DPG docs

Basic samples guidance is illustrated in: https://github.com/Azure/azure-sdk-for-net/tree/main/sdk/template/Azure.Template/samples.

Key features of Azure SDK .NET Samples include:

• samples directory: azure-sdk-for-net/tree/main/sdk/<service>/<library>/samples: see example
• README.md providing a TOC for all available samples under <library>/samples: see example
• Sample<i>_<scenario>.md files describing the scenario the sample is illustrating under <library>/samples: see example
• test/samples directory: azure-sdk-for-net/tree/main/sdk/<service>/<library>/tests/samples: see example
• tests inheriting from TestFramework SamplesBase that implement all sample code under <library>/tests/samples: see example
• snippet regions identifying snippets to show in README and Samples files: see example

A nice example of a service library that follows the .NET samples guidance is MetricsAdvisor: https://github.com/Azure/azure-sdk-for-net/tree/main/sdk/metricsadvisor/Azure.AI.MetricsAdvisor

[kinelski]

@annelo-msft We had no particular reason other than my lack of understanding about how the Test Framework works. I created an issue to track this work: https://github.com/Azure/azure-sdk-for-net/issues/27590

[annelo-msft]

We don’t need to block on Consolidate smoke tests and best practices with DPG partner tools - we can loop back and update this after that merges.