Help: Rework docs to prioritize FetchContent_MakeAvailable()

FetchContent_MakeAvailable() should be strongly preferred over
the direct use of FetchContent_Populate(). The latter was the only
method available before CMake 3.14, but the former is simpler and
offers more functionality. Restructure the docs and improve the
wording to steer the reader toward `FetchContent_MakeAvailable()`
as the preferred approach.

Also take the opportunity to pull out the variables that influence
behavior into their own section. They were easily missed when
buried in the docs for the FetchContent_Populate() command.

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