cmake: use standard GNU install directories for docs #1286

Job	Run time
build (x64, release, windows-2019, ON, ON, Visual Studio 16 2019, v142, vs2019, v142-x64, ON)	5m 22s
cron	0s
build (ubuntu-latest, default, asciidoctor, disabled, select)	3m 21s
build (ubuntu-latest, default, disabled, poll)	2m 18s
build (ubuntu-latest, android, android-ndk-r25, disabled)	3m 37s
build (ubuntu-latest, coverage, libkrb5-dev libnorm-dev libpgm-dev libgnutls28-dev lcov, enabled,...	2m 56s
build (ubuntu-latest, valgrind, valgrind libgnutls28-dev, enabled)	8m 11s
build (ubuntu-latest, cmake, libsodium, enabled, cmake libsodium-dev, enabled)	3m 30s
build (ubuntu-latest, cmake, libsodium, enabled, enabled, cmake libsodium-dev libkrb5-dev, enabled)	3m 39s
build (ubuntu-latest, cmake, enabled, cmake clang-format-11, 1)	15s
build (ubuntu-latest, default, libkrb5-dev libnorm-dev libpgm-dev libgnutls28-dev libsodium-dev l...	2m 58s
build (ubuntu-latest, default, libkrb5-dev libnorm-dev libpgm-dev libgnutls28-dev libsodium-dev l...	3m 4s
build (ubuntu-latest, default, libkrb5-dev libnorm-dev libpgm-dev libgnutls28-dev libsodium-dev l...	2m 51s
build (ubuntu-latest, abi-compliance-checker, abi-dumper abi-compliance-checker, disabled)	3m 5s
build (ubuntu-latest, cmake, clang-tidy clang-tools, enabled, clang++)	3m 51s
build (macos-latest, default, automake autoconf libtool, enabled)	3m 31s
build (macos-latest, default, automake autoconf libtool libsodium, libsodium, disabled)	3m 9s
	55m 38s

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