[doc] html_unescape: Create html.unescape extension and use it for no-break space #2342

Job	Run time
host-windows-cross	55s
docs	52s
host-linux	40s
client (gcc, g++, xdg-shell, Release)	1m 3s
host-windows-native	1m 29s
client (gcc, g++, xdg-shell, Debug)	56s
module	15s
client (gcc, g++, libdecor, Release)	1m 28s
obs (gcc)	38s
client (gcc, g++, libdecor, Debug)	59s
obs (clang)	39s
client (clang, clang++, xdg-shell, Release)	1m 32s
client (clang, clang++, xdg-shell, Debug)	1m 5s
client (clang, clang++, libdecor, Release)	1m 30s
client (clang, clang++, libdecor, Debug)	1m 3s
	15m 4s

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