Documentation updates: Sites reference and assertNumQueries().

[garrypolley]

This PR contains two documentation updates.

1. To address the request here: Removed outdated examples of flatpages on Django. #11667 -- my goal is minimal changes while still keep the good example in place.
2. Hopefully make the assertNumQueries be more obvious that you can pass using as a kwarg. I got confused when I read the docs and the example talked about the lambda for the call.

[garrypolley]

How do I run the docs locally and get them to look like the current styles on djangoproject? When I run locally they look like the previous styles with the greens.

[garrypolley]

I do not want to seem impatient or rude. @carltongibson Are there updates I should make to this PR? I made this PR based on the feedback of #11667

[felixxm]

@garrypolley Thanks 👍