Update prompt and model for watsonx summarization

[jwm4]

Context: https://github.ibm.com/watson-engagement-advisor/wea-backlog/issues/60756

Major changes:

• Model used for the watsonx kit is now Granite v2 instruct.
• Prompt template used for the watsonx kit is updated based on input from Andrew Freed a few months ago with additional re-engineering for Granite v2 instruct. (Sorry this has taken so long to get done!)
• README updates:
  • More details about the prompt template being used + how and why users might want to do their own prompt engineering.
  • Technical details were moved to the watsonx.ai example section since that is the focus now.
  • README now uses the phrase "watsonx.ai" more often instead of just "watsonx" since there are an assortment of watsonx branded products now.
  • Sample screenshot updated to show actual Granite v2 instruct output.

[jwm4]

The image in the PR came from prod, so I think I will stick with it since I think it is closest to what new users will see.