docs: updated env in readme local setup

[Priyanka-Microsoft]

Purpose

• ...
  This pull request updates the local deployment documentation to clarify how to set up the .env file for the backend. It adds instructions for both manual and automated provisioning scenarios.

Documentation improvements:

• Clarified that the .env file for src\backend should be updated with values from Azure App Service environment variables.
• Added instructions that if resources are provisioned using azd provision or azd up, the .env file is automatically generated in .azure/<env-name>/.env, and provided guidance on how to find the <env-name>.

Does this introduce a breaking change?

• Yes
• No

How to Test

• Get the code

git clone [repo-address]
cd [repo-name]
git checkout [branch-name]
npm install

• Test the code

What to Check

Verify that the following are valid

• ...

Other Information

[github-actions]

🎉 This PR is included in version 2.2.0 🎉

The release is available on GitHub release

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