Docs: Format Space Cadet documentation (#3540)

* Format Space Cadet documentation

* Fix page filenames

* Move Caveats section up

docs/feature_space_cadet_shift_enter.md

@@ -0,0 +1,31 @@
+# Space Cadet Shift Enter
+
+Based on the [Space Cadet Shift](feature_space_cadet_shift.md) feature. Tap the Shift key on its own, and it behaves like Enter. When held, the Shift functions as normal.
+
+## Usage
+
+Replace any Shift key in your keymap with `KC_SFTENT` (Shift, Enter), and you're done.
+
+## Keycodes
+
+|Keycode    |Description                             |
+|-----------|----------------------------------------|
+|`KC_SFTENT`|Right Shift when held, Enter when tapped|
+
+## Caveats
+
+As with Space Cadet Shift, this feature may conflict with Command, so it should be disabled in your `rules.mk` with:
+
+```make
+COMMAND_ENABLE = no
+```
+
+This feature also uses the same timers as Space Cadet Shift, so using them in tandem may produce strange results.
+
+## Configuration
+
+By default Space Cadet assumes a US ANSI layout, but if you'd like to use a different key for Enter, you can redefine it in your `config.h`:
+
+|Define      |Default |Description                                     |
+|------------|--------|------------------------------------------------|
+|`SFTENT_KEY`|`KC_ENT`|The keycode to send when the Shift key is tapped|