docs: Document updated for .env setup steps for local development with azd

[UtkarshMishra-Microsoft]

Purpose

Updated the docs/DeploymentGuide.md to explain where the .env file is generated when using azd provision or azd up, and provided clear instructions on copying its contents to the backend .env file. Also added guidance for setting BACKEND_API_URL and FRONTEND_SITE_NAME for local development.

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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