Help: Modernize PackageConfigHelpers example

The example shown in the module documentation for CMakePackageConfigHelpers was showing its age, by:

1. Hardcoding directory prefixes (etc/, lib/, etc.) instead of using GNUInstallDirs
2. Handwaving set() commands (incorrectly!) as:
   set(VAR dir/ ... CACHE )
   which should, at the very least, be:
   set(VAR dir/ CACHE ... )
3. Installing CMake configuration files to lib/Foo/cmake/, when current practice favors ${CMAKE_INSTALL_LIBDIR}/cmake/Foo/

This updated example addresses all of those issues.

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• bfa61ccf - Help: Modernize PackageConfigHelpers example

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Do: stage

assigned to @brad.king

added area:doc label

changed milestone to %3.26.0

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Do: merge

mentioned in commit 62483b9b

merged

removed workflow:nightly-testing label