Update README with prerequisites and setup instructions

[rodrigobr-msft]

This pull request updates the instructions in the samples/python/copilotstudio-client/README.md file to improve clarity and accuracy for setting up an Application Registration in Entra ID. The steps have been reorganized, clarified, and expanded to ensure users can follow them more easily and avoid common pitfalls.

Key improvements to setup instructions:

• Clarified and reorganized the step-by-step process for creating a Native Client Application Identity in Entra ID, including consistent numbering and clearer language.
• Added instructions to verify that the "Enable the following mobile and desktop flows" toggle is set to "Yes" under Authentication > Advanced settings.
• Improved the description of how to add delegated permissions for the Power Platform API, specifying the correct sequence in the Azure Portal.

Minor updates:

• Added a blank line at the end of the README for formatting consistency.