Guidance to implement `Get` for local-deploy

**Is your feature request related to a problem? Please describe.**
The current [example](https://github.com/Azure/bicep/blob/main/docs/experimental/local-deploy-dotnet-quickstart.md) in the docs illustrates how you can get started with developing your own custom Bicep extension.

However, when I try to use the `existing` keyword, it tells me:

```plaintext
{"errorData":{"error":{"code":"NotImplemented","message":"Operation '.Get' has not been implemented.","target":"","details":[]}}}
```

It would be helpful to include examples how you can implement `Get`.

**Describe the solution you'd like**
A clear and concise description of what you want to happen.

Start with docs/experimental/local-deploy-dotnet-quickstart.md, which is the example identified in the issue, and reproduce the existing-keyword error for a custom Bicep extension. Document clear examples showing how to implement Get, and verify that the quickstart explains the expected behavior without producing the NotImplemented response.

Written by the indexing model from the issue text.