Clarify how to set $Context.Input in Durable Functions docs

A customer (from IcM 277661225) followed [this documentation for handling External Events](https://docs.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-external-events?tabs=powershell) and was confused about how to set $Context.Input. We clarified that they would need to set the `-InputObject` parameter when they started an orchestration (example below) in the starter function.

```
$InstanceId = Start-DurableOrchestration -FunctionName $FunctionName -InputObject $Input
```

Adding an example with `-InputObject` in the docs would help customers in the future understand how to pass information between starter and orchestrator functions.

cc: @AnatoliB @davidmrdavid

Start with the linked Durable Functions documentation for handling External Events and review the starter-function example. Add an example showing the -InputObject parameter when starting the orchestration, then verify that the example clearly explains how the value becomes $Context.Input in the orchestrator.

Written by the indexing model from the issue text.