[Keymap] Add new TADA68 keymap (#6938)

* Add new TADA68 keymap

* Remove unnecessary backlashes

* Change from MacOS specific to generic volume keycodes

keyboards/tada68/keymaps/peippo/readme.md

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+# TADA68 ISO keymap by peippo
+
+Nordic ISO layout with faster access to cursor movement keys and brackets.
+
+## Keymap notes
+
+-   A few MacOS specific shortcuts, but also has a key dedicated for swapping Alt and GUI buttons for Windows use.
+-   Default layer has a Grave Escape which outputs Escape when pressed normally, and `§` when Shift or Command are held. I like to setup Command+Esc to moving focus to next window (MacOS preferences, Keyboard / Shortcuts / Keyboard).
+-   `Caps Lock` or `FN` can be held to access the modifier layer.
+
+## Modifier layer
+
+-   Additional cursor movement keys on `J/K/L/I`, and Page Up/Page Down on `U/O`.
+-   Easier access to [] & {} (w/ Shift) on `8/9` (MacOS).
+-   Show desktop on `D` (MacOS).
+-   Screen capture tool on `S` (MacOS).
+-   Media shortcuts on Arrow keys & Page Up/Page Down.
+-   MacOS/Windows key for swapping Alt and GUI on both sides.
+-   Backlighting adjustment (Toggle on `B`, increase/decrease on `V/B`).
+
+### Build
+
+To build the firmware, run `make tada68:peippo:flashbin`.
+Detailed instructions at https://github.com/qmk/qmk_firmware/tree/master/keyboards/tada68