Draft of the document on "How to mock extension method in mgmt SDKs"

[ArcturusZhang]

Related issue: #3493

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[ArcturusZhang]

@m-nash @KrzysztofCwalina @tg-msft could you also have a review on this to see if anything is missing in this doc?

Related implementations: #3483 and Azure/azure-sdk-for-net#37927

[scottaddie]

@ArcturusZhang Let's hold off on merging this PR. The content is better suited for learn.microsoft.com.

[ArcturusZhang]

@ArcturusZhang Let's hold off on merging this PR. The content is better suited for learn.microsoft.com.

Yes we are not targeting to merge this, just use this PR to collect feedbacks and comments
Also we will release a few beta versions for some libraries and the testers might need a document, this is also for that purpose too

[ArcturusZhang]

I also added a section for NSubstitute

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[microsoft-github-policy-service]

Hi @ArcturusZhang. Thank you for your contribution. Since there hasn't been recent engagement, we're going to close this out. Feel free to respond with a comment containing "/reopen" if you'd like to continue working on these changes. Please be sure to use the command to reopen or remove the "no-recent-activity" label; otherwise, this is likely to be closed again with the next cleanup pass.