Add 'nix edit' manpage

src/nix/edit.md

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+R""(
+
+# Examples
+
+* Open the Nix expression of the GNU Hello package:
+
+  ```console
+  # nix edit nixpkgs#hello
+  ```
+
+* Get the filename and line number used by `nix edit`:
+
+  ```console
+  # nix eval --raw nixpkgs#hello.meta.position
+  /nix/store/fvafw0gvwayzdan642wrv84pzm5bgpmy-source/pkgs/applications/misc/hello/default.nix:15
+  ```
+
+# Description
+
+This command opens the Nix expression of a derivation in an
+editor. The filename and line number of the derivation are taken from
+its `meta.position` attribute. Nixpkgs' `stdenv.mkDerivation` sets
+this attribute to the location of the definition of the
+`meta.description`, `version` or `name` derivation attributes.
+
+The editor to invoke is specified by the `EDITOR` environment
+variable. It defaults to `cat`. If the editor is `emacs`, `nano` or
+`vim`, it is passed the line number of the derivation using the
+argument `+<lineno>`.
+
+)""