Feature an example of InMemoryRunner() usage in the "Running Agents" doc

**Is your feature request related to a problem? Please describe.**

It's not easy to find a basic example of running an agent using InMemoryRunner(), especially in the place you'd expect to find it ("Running Agents")

**Describe the solution you'd like**

It would be great to include some short snippet showing the new usage of InMemoryRunner():

**Describe alternatives you've considered**

You can find InMemoryRunner in other code snippets if you know that's what you're looking for, but it's not obvious from "Running an Agent" what to look for.