docs: updated the env example and local setup guide

[Dhruvkumar-Microsoft]

Purpose

This pull request updates documentation and environment configuration to clarify setup steps and align with new Azure service roles and model deployments. The main themes are improvements to deployment instructions, expanded role assignment guidance, and updates to environment variables for new model versions.

Documentation improvements:

• Updated the local deployment and debugging section header in docs/DeploymentGuide.md to better reflect setup steps.
• Clarified instructions for updating the .env file, specifying the use of Backend Container app environment variables instead of App Service.

Role assignment and permissions:

• Expanded the description of role assignments in Bicep deployments to include AOAI, Storage account, Search service, and Cosmos services, with instructions for using resource groups not deployed by the user.
• Added example Azure CLI commands for assigning roles: "Azure AI Developer," "Cognitive Services OpenAI User," "Search Index Data Contributor," and "Storage Blob Data Contributor."

Environment variable updates:

• Updated model names, deployment names, and API versions in src/backend/.env.sample to use the new gpt-4.1-mini model and related configuration; also revised reasoning model name and clarified variable names for Azure credentials.

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