Docs: hosting-only Azure Functions apps get no agent routes without an orchestration trigger

In a .NET isolated Azure Functions app that hosts agents only through ConfigureDurableAgents(...) and defines no Durable orchestrator of its own, the built-in agent routes (/api/agents/{agentName}/run) never appear at startup.

The reason: the worker SDK indexes the Durable extension from triggers discovered in the app assembly, and with no [OrchestrationTrigger] in the app there is nothing to index, so the hosting package's routes are never registered. The current READMEs (hosting package and the Azure Functions sample) do not mention this, so a hosting-only app fails silently with an empty route table.

I have a small docs patch ready that adds a short note plus a minimal placeholder trigger to both READMEs: https://github.com/he-yufeng/agent-framework-durable-extension/tree/docs/azurefunctions-trigger-discovery

Happy to open it as a PR once external PRs are enabled for this repo, or feel free to take the text over directly if that is easier.

Start with the hosting package README and the Azure Functions sample README mentioned in the issue, then review the described trigger-discovery behavior. Add the short note and minimal placeholder trigger to both READMEs; done means hosting-only apps have documented guidance for getting the built-in agent routes registered.