Document OpenTelemetry support

[andystaples]

Need to add public-facing documentation for getting started with OpenTelemetry in PowerShell functions before wider feature announcement. Documentation will be a combination of environment variables for config found here: https://github.com/open-telemetry/opentelemetry-dotnet/blob/main/src/OpenTelemetry.Exporter.OpenTelemetryProtocol/README.md#exporter-configuration
Module description + examples found in the module repo here: https://github.com/Azure/azure-functions-powershell-opentelemetry
and existing best practices for function apps

[Francisco-Gamino]

/cc @RohitRanjanMS @andystaples