Meson

docs: move docs into nixd/ subfolder #1757

Job	Run time
nix-build-develop-gcc (none, release, true)	3m 7s
nix-build-develop-clang (none, release, true)	2m 49s
nix-build-develop-clang (none, release, false)	2m 52s
nix-build-develop-clang (none, debug, true)	2m 29s
nix-build-develop-gcc (none, release, false)	3m 11s
nix-build-develop-clang (none, debug, false)	2m 36s
nix-build-develop-gcc (none, debug, true)	2m 59s
nix-build-develop-gcc (none, debug, false)	3m 6s
nix-build-develop-gcc (undefined, release, true)	6m 40s
nix-build-develop-gcc (undefined, release, false)	6m 32s
nix-build-develop-gcc (undefined, debug, true)	3m 27s
nix-build-develop-gcc (undefined, debug, false)	3m 26s
	43m 14s

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