keyboardio/atreus: Add ardumont keymap (#12479)

keyboards/keyboardio/atreus/keymaps/ardumont/readme.md

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+![ardumont layout](https://i.imgur.com/tuk64AI.png)
+
+# ardumont layout
+
+This is a qwerty layout for the keyboardio/atreus. For the most part, it's a
+straightforward layout. Only the modifiers key are not organized like the default
+layout. Those modifier keys are mostly symmetrically configured.
+
+The idea is that when using a modifier key, some users may expect to use both side of
+the keyboard symmetrically (as in emacs, qutebrowser, terminal/tmux).
+
+Exceptions to the rules are the `Escape` and the `Super` which only exists on the left
+Side of the keyboard. Those are mostly unused.
+
+Note that some keys are not present in the keyboard (e.g `Tab`, `Enter`, ...) since they
+are available from the legacy terminal bindings:
+
+- Ctrl-i: Tab
+- Ctrl-m: Enter
+- Ctrl-h: Backspace
+- Ctrl-d: Del
+- Ctrl-n: Down
+- Ctrl-p: Up