Doc: EXTENSIONS.md, Cppcheck, INSTALLATION.md refactoring (#199) #82

Job	Run time
build-linux / build	24m 16s
build-osx / build-x64	1m 11s
build-windows / build	4m 6s
build-osx / build-universal	5m 19s
build-qnap / build	2m 29s
build-synology / build	10m 24s
build-osx / combine-osx-artifacts	5s
repack-qnap / build-linux	0s
repack-qnap / repack	31s
generate-signatures	32s
make-testing-release	1m 41s
	50m 34s

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