Help: Add placeholder documents for CTest Script variables

Provide a minimal explanation of each variable by simply naming
the corresponding ctest(1) setting and linking to the ctest(1)
manual.
Help: Convert CTest Script variable references to cross-references

In the ctest(1) manual and ctest_start() command documentation, convert
mentions of CTest Script variables to cross-references so they will be
rendered as links.
Merge topic 'doc-ctest-variables'

34c6fd3a Help: Convert CTest Script variable references to cross-references
de710f37 Help: Add placeholder documents for CTest Script variables