Mock service injection for integration tests topic

[guardrex]

Fixes #7153

Internal Review Topic

• Adds content to the topic to cover injection of mock services with ConfigureTestServices.
• Adds an example of injecting a simple mocked service into the sample app.

cc/ @lambdakris You won't be able to see the internally-built doc; but if you flip over to the diff, you'll see the enhancement. I think this would have been enough to show you the way to go, but let me know if not.

[scottaddie]

@guardrex The snippet displays correctly in the preview page. Try upgrading to the latest version of DocFX, if you haven't already.

[guardrex]

I did. No 🎲🎲.

[Rick-Anderson]

@javiercn please review

[guardrex]

@Rick-Anderson I updated the versions. I reviewed the text updates ... look'in good 👍, and I checked the sample again ...

Total tests: 14. Passed: 14. Failed: 0. Skipped: 0.
Test Run Successful.
Test execution time: 8.7050 Seconds

Shall we go ahead and merge?