Add 'nix bundle' manpage

Fixes #4375.

src/nix/bundle.cc

@@ -40,14 +40,11 @@ struct CmdBundle : InstallableCommand
         return "bundle an application so that it works outside of the Nix store";
     }
 
-    Examples examples() override
+    std::string doc() override
     {
-        return {
-            Example{
-                "To bundle Hello:",
-                "nix bundle hello"
-            },
-        };
+        return
+          #include "bundle.md"
+          ;
     }
 
     Category category() override { return catSecondary; }

src/nix/bundle.md

@@ -0,0 +1,32 @@
+R""(
+
+# Examples
+
+* Bundle Hello:
+
+  ```console
+  # nix bundle nixpkgs#hello
+  # ./hello
+  Hello, world!
+  ```
+
+* Bundle a specific version of Nix:
+
+  ```console
+  # nix bundle github:NixOS/nix/e3ddffb27e5fc37a209cfd843c6f7f6a9460a8ec
+  # ./nix --version
+  nix (Nix) 2.4pre20201215_e3ddffb
+  ```
+
+# Description
+
+`nix bundle` packs the closure of the [Nix app](./nix3-run.md)
+*installable* into a single self-extracting executable. See the
+[`nix-bundle` homepage](https://github.com/matthewbauer/nix-bundle)
+for more details.
+
+> **Note**
+>
+> This command only works on Linux.
+
+)""