doc: point manual contributing guides to devmode's README (#411326)

There's quite a bit of pingpong redirection with Nixpkgs and NixOS
manual utilities. Since devmode was lacking a README, the descriptive
text is moved there and it's referenced by both manuals.

doc/README.md

···
 
 #### `devmode`
 
-The shell in the manual source directory makes available a command, `devmode`.
-It is a daemon, that:
-1. watches the manual's source for changes and when they occur — rebuilds
-2. HTTP serves the manual, injecting a script that triggers reload on changes
-3. opens the manual in the default browser
+Use [`devmode`](../pkgs/by-name/de/devmode/README.md) for a live preview when editing the manual.
 
 ### Testing redirects
 

nixos/doc/manual/contributing-to-this-manual.chapter.md

···
 /**/.direnv
 ```
 
-You might want to also use [`devmode`](https://github.com/NixOS/nixpkgs/blob/master/doc/README.md#devmode) while editing the manual.
+### `devmode` {#sec-contributing-devmode}
+
+Use [`devmode`](https://github.com/NixOS/nixpkgs/blob/master/pkgs/by-name/de/devmode/README.md) for a live preview when editing the manual.
 
 ## Testing redirects {#sec-contributing-redirects}
 

nixos/doc/manual/redirects.json

···
   "sec-contributing-development-env": [
     "index.html#sec-contributing-development-env"
   ],
+  "sec-contributing-devmode": [
+    "index.html#sec-contributing-devmode"
+  ],
   "sec-mattermost": [
     "index.html#sec-mattermost"
   ],

pkgs/by-name/de/devmode/README.md

···
+# `devmode`
+
+`devmode` is a daemon, that:
+1. watches the manual's source for changes and when they occur — rebuilds
+2. HTTP serves the manual, injecting a script that triggers reload on changes
+3. opens the manual in the default browser