Add doc for the envs supported in azure-core

[xiangyan99]

Description

Please add an informative description that covers that changes made by the pull request and link all relevant issues.

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• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

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Testing Guidelines

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[Copilot]

Pull request overview

Adds/centralizes documentation for environment variables used by azure-core (settings, tracing, HTTP pipeline behavior, and proxy configuration) and surfaces that information in developer-facing docs and docstrings.

Changes:

• Added a new ENVIRONMENT_VARIABLES.md reference doc enumerating supported environment variables.
• Added an “Environment Variables” section to CLIENT_LIBRARY_DEVELOPER.md and cross-referenced it from the proxy settings discussion.
• Updated docstrings in Settings, transports, and pipeline policies to explicitly mention relevant environment variables.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
sdk/core/azure-core/ENVIRONMENT_VARIABLES.md	New reference document for azure-core environment variables.
sdk/core/azure-core/CLIENT_LIBRARY_DEVELOPER.md	Adds an Environment Variables section and updates proxy-settings docs to include NO_PROXY and link to env var info.
sdk/core/azure-core/azure/core/settings.py	Extends Settings documentation to include AZURE_SDK_CLOUD_CONF and a summary of settings-related env vars.
sdk/core/azure-core/azure/core/pipeline/transport/_requests_basic.py	Documents proxy-related env vars honored when use_env_settings=True.
sdk/core/azure-core/azure/core/pipeline/transport/_aiohttp.py	Documents proxy-related env vars honored when use_env_settings=True.
sdk/core/azure-core/azure/core/pipeline/policies/_universal.py	Documents AZURE_HTTP_USER_AGENT and AZURE_SDK_LOGGING_MULTIRECORD env vars in policy docstrings.

[xiangyan99]

/check-enforcer override