Add guideline to use example urls

What does this MR do and why?

As a general practice, we should avoid including actual URLs in open-source project’s code examples, rspecs test cases and templates since they may point to proprietary resources or external services and can lead to litigations and hacks.

As an example, An adversary could see the URL my-duo-workflow-service-url.com in our code, purchase it, and create a phishing site. Since our project is open-source and used by many contributors outside the company, this could lead to potential hack. I would like to recommend adding the instruction to use localhost/dev sites for testing in our Testing best practices guide.

This MR is adding the general guideline to use example.com as test urls in rspec.

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