docs: updated the env step in deployment guide readme

[Priyanka-Microsoft]

Purpose

• ...
  This pull request updates the deployment guide to clarify the process of configuring your environment, especially regarding the generation and location of the .env file when using Azure Developer CLI (azd). The instructions now account for scenarios where resources are provisioned with azd, making the setup process clearer for users.

Deployment documentation improvements:

• Updated the environment configuration steps in docs/DeploymentGuide.md to explain that if resources are provisioned using azd provision or azd up, the .env file is automatically generated in the .azure/<env-name>/.env directory, and provided guidance on how to determine the current environment name using azd env list.
• Adjusted the step order and clarified that users must still ensure APP_ENV is set to "dev" in their .env file, regardless of how the file was created.

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

• I have built and tested the code locally and in a deployed app
• For frontend changes, I have pulled the latest code from main, built the frontend, and committed all static files.
• This is a change for all users of this app. No code or asset is specific to my use case or my organization.

Other Information

[github-actions]

🎉 This PR is included in version 1.6.1 🎉

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