Add CMAKE variable list to F Prime docs

[timcanham]

F´ Version	3.x
Affected Component	n/a

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Feature Description

There are a handy set of CMAKE defined variables like ${FPRIME_PROJECT_ROOT} that make the CMakeList.txt pathnames cleaner. Add a table of them to the F Prime docs with some examples.

Rationale

Makes writing CMakeList.txt files easier.

[LeStarch]

This is a web linking issue, this is documented.