[doc] html_unescape: Create html.unescape extension and use it for no-break space #2343

Job	Run time
host-linux	42s
docs	49s
host-windows-cross	59s
client (gcc, g++, xdg-shell, Release)	1m 10s
client (gcc, g++, xdg-shell, Debug)	1m 3s
host-windows-native	1m 32s
module	30s
obs (gcc)	46s
client (gcc, g++, libdecor, Release)	1m 4s
obs (clang)	57s
client (gcc, g++, libdecor, Debug)	1m 10s
client (clang, clang++, xdg-shell, Release)	1m 28s
client (clang, clang++, xdg-shell, Debug)	1m 23s
client (clang, clang++, libdecor, Release)	1m 26s
client (clang, clang++, libdecor, Debug)	1m 21s
	16m 20s

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