docs: bring back flat widget directory structure

docs/widgets/btn.md

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+# Button (lv_btn)
+
+## Overview
+
+Buttons have no new features compared to the [Base object](/widgets/obj). They are useful for semantic purposes and have slightly different default settings.
+
+Buttons, by default, differ from Base object in the following ways:
+- Not scrollable
+- Added to the default group
+- Default height and width set to `LV_SIZE_CONTENT`
+
+## Parts and Styles
+- `LV_PART_MAIN` The background of the button. Uses the typical background style properties.
+
+## Usage
+
+There are no new features compared to [Base object](/widgets/obj).
+
+## Events
+- `LV_EVENT_VALUE_CHANGED` when the `LV_OBJ_FLAG_CHECKABLE` flag is enabled and the object is clicked. The event happens on transition to/from the checked state.
+
+
+Learn more about [Events](/overview/event).
+
+## Keys
+Note that the state of `LV_KEY_ENTER` is translated to `LV_EVENT_PRESSED/PRESSING/RELEASED` etc.
+
+See the events of the [Base object](/widgets/obj) too.
+
+Learn more about [Keys](/overview/indev).
+
+## Example
+```eval_rst
+
+.. include:: ../../../examples/widgets/btn/index.rst
+
+```
+
+## API
+
+```eval_rst
+
+.. doxygenfile:: lv_btn.h
+  :project: lvgl
+
+```