Help: Rework docs to prioritize FetchContent_MakeAvailable()

This is preparation for some upcoming expanded functionality for the FetchContent_MakeAvailable() command and other performance improvements. The change here makes FetchContent_MakeAvailable() more prominent, steering the reader away from the old, more manual method calling FetchContent_Populate() directly.

As part of the reorganisation, the variables have been moved out to their own section. Anecdotal evidence has suggested that awareness of these variables among users could be better. They were previously buried in the docs for FetchContent_Populate(), but they logically belong at the module level, not as part of a particular command. With focus moving away from FetchContent_Populate() and onto FetchContent_MakeAvailable(), these variable docs needed to move out of their old location anyway.