[Test Proxy] Clarify relevant documentation in testing guides

[mccoyp]

Description

Python's test proxy documentation currently suggests that users may have to set up the test proxy manually by using azure-sdk-tools's instructions (a partner team recently raised this issue). This PR clarifies that the documentation in azure-sdk-tools is technical reference and is unnecessary for test configuration. Related: Azure/azure-sdk-tools#6368

All SDK Contribution checklist:

• 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.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.