Add example for package extensions #4468

Job	Run time
testinstall - - ubuntu-latest	5m 57s
teststandard - - ubuntu-latest	35m 28s
testmanuals - - ubuntu-latest	8m 30s
testexpect testmockpkg testspecial test-compile testlibgap testkernel - - ubuntu-latest	4m 46s
testpackages testinstall-loadall - - ubuntu-latest	16m 17s
makemanuals testmakeinstall - GAPPREFIX=/tmp/gapprefix CONFIGFLAGS="--prefix=/tmp/gapprefix" NO_COVERAGE=1 - ubuntu-latest	7m 56s
teststandard - ABI=32 CONFIGFLAGS="" - ubuntu-20.04	26m 4s
testinstall - HPCGAP=yes ABI=64 - ubuntu-latest	35m 35s
testmockpkg testinstall - BOOTSTRAP_MINIMAL=yes - macos-latest	7m 38s
testbugfix - CONFIGFLAGS="--enable-memory-checking" - ubuntu-latest	29m 38s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=64 BUILDDIR=out-of-tree CONFIGFLAGS="--enable-valgrind" - ubuntu-latest	6m 26s
testbuildsys testmockpkg testinstall - NO_COVERAGE=1 ABI=32 BUILDDIR=out-of-tree CONFIGFLAGS="" - ubuntu-20.04	4m 16s
testinstall - JULIA=yes CONFIGFLAGS="--enable-debug --disable-Werror" - ubuntu-20.04	15m 21s
testmockpkg testinstall - - windows-2019	18m 2s
Send Slack notification on status change	0s
	3h 41m 54s

Provide feedback