Meson

docs: move docs into nixd/ subfolder #1758

Job	Run time
nix-build-develop-clang (none, release, true)	2m 57s
nix-build-develop-gcc (none, release, true)	3m 3s
nix-build-develop-clang (none, release, false)	2m 55s
nix-build-develop-gcc (none, release, false)	3m 7s
nix-build-develop-clang (none, debug, true)	2m 37s
nix-build-develop-gcc (none, debug, true)	3m 2s
nix-build-develop-clang (none, debug, false)	2m 37s
nix-build-develop-gcc (none, debug, false)	2m 58s
nix-build-develop-gcc (undefined, release, true)	6m 39s
nix-build-develop-gcc (undefined, release, false)	6m 34s
nix-build-develop-gcc (undefined, debug, true)	3m 33s
nix-build-develop-gcc (undefined, debug, false)	3m 37s
	43m 39s

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