docs: add backend verification note for backend setup

[jao-o-garcia]

Backend verification note

This adds a curl test to verify that the ChatKit server and its memory store are correctly configured before working on the frontend.

I ran into this while trying to confirm my backend was functioning prior to UI development. The documentation does not clearly specify what payload should be sent to /chatkit. Many AI suggestions recommended sending:

curl -X POST http://localhost:8000/chatkit \
  -H "Content-Type: application/json" \
  -d '{}'

However, this results in an Internal Server Error, which can misleadingly make it seem like the server or installation is broken.

I determined the required parameters by using one of the ChatKit customer-service examples and logging the actual request payload. Including this verification step should help developers quickly confirm that their backend and memory store are properly set up and avoid unnecessary debugging.

I've also added a chore where the document gives the developer the option to choose uv.