docs: Readme updates

[Shreyas-Microsoft]

Purpose

This pull request mainly improves the configuration handling for authentication in the frontend and updates the deployment documentation to clarify resource cleanup steps. The most important changes are grouped below.

Frontend Authentication Configuration:

• Added new MSAL-specific authentication environment variables (REACT_APP_MSAL_AUTH_CLIENTID, REACT_APP_MSAL_AUTH_AUTHORITY, REACT_APP_MSAL_REDIRECT_URL, REACT_APP_MSAL_POST_REDIRECT_URL) to the config object in main.jsx to support Microsoft Authentication Library (MSAL) flows. [1] [2]
• Ensured that these new variables default to the same values as their existing counterparts if not set, improving flexibility for different authentication providers. [1] [2]

Documentation Updates:

• Added a note in DeploymentGuide.md about using azd down or following the Delete Resource Group Guide for purging resources and cleaning up after deployment, making the cleanup process clearer for users.

Does this introduce a breaking change?

• Yes
• No

Golden Path Validation

• I have tested the primary workflows (the "golden path") to ensure they function correctly without errors.

Deployment Validation

• I have validated the deployment process successfully and all services are running as expected with this change.

What to Check

Verify that the following are valid

• ...

Other Information

[github-actions]

🎉 This PR is included in version 1.0.1 🎉

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