CODEOWNERS: add ryantm to /nixos/doc

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.github
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.github/CODEOWNERS
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/nixos/default.nix @nbp @infinisil
/nixos/lib/from-env.nix @nbp @infinisil
/nixos/lib/eval-config.nix @nbp @infinisil
+
/nixos/doc @ryantm
/nixos/doc/manual/configuration/abstractions.xml @nbp
/nixos/doc/manual/configuration/config-file.xml @nbp
/nixos/doc/manual/configuration/config-syntax.xml @nbp