[BUG] replace question answering demo with the new PDF reader tutorial demo

[amirfz]

Describe the bug
currently there's a "question answering" demo that references slack app util and other things that no longer exist.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://github.com/Aggregate-Intellect/sherpa/tree/main/demo/question_answering
2. Try to run this locally (I assumed this is the PDF reader tutorial)
3. I got a slackapp util error and was confused for a while before I realized that this is not the PDF qa tutorial

Expected behavior
This is no longer a good demo since the library has changed and now our primary tutorial is the PDF qa one. However, the PDF tutorial only exists in the docs and not as a runnable demo. We should delete this demo and create a new one based on the PDF reader.

Screenshots

Environment
n/a

Additional context
n/a