Start work on documenting the desktop-ui kitten

Author: kovidgoyal

docs/kittens/desktop-ui.rst

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+Using terminal programs to provide Linux desktop components
+===============================================================
+
+.. only:: man
+
+    Overview
+    --------------
+
+.. versionadded:: 0.43.0
+
+Power users of terminals on Linux also often like to use bare bones window
+managers instead of full fledged desktop environments. This kitten helps
+provide parts of the desktop environment that are missing from such setups,
+and does so using keyboard friendly, terminal first UI components. Some of its
+features are:
+
+  * Replace the typical File Open/Save dialogs used in GUI programs with the
+    fast and keyboard centric :doc:`choose-files </kittens/choose-files>` kitten
+    running in a semi-transparent kitty overlay.
+
+  * Allow simple command line based management of the desktop color scheme
+    light/dark modes.