Add documentation for APPFL+MONAI integration to do federated learning

[Zilinghan]

Description

In this pull request, I added documentation for the APPFL+MONAI integration to do federated learning.

Checks

• Avoid including large-size files in the PR.
• Clean up long text outputs from code cells in the notebook.
• For security purposes, please check the contents and remove any sensitive info such as user names and private key.
• Ensure (1) hyperlinks and markdown anchors are working (2) use relative paths for tutorial repo files (3) put figure and graphs in the ./figure folder
• Notebook runs automatically ./runner.sh -t <path to .ipynb file>

[holgerroth]

Thanks for your contribution!