feat(docs): migrate from .md to .rst (#4129)

docs/widgets/slider.rst

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+.. _slider:
+
+Slider (lv_slider)
+==================
+
+Overview
+********
+
+The Slider object looks like a `Bar </widgets/bar>`__ supplemented with
+a knob. The knob can be dragged to set a value. Just like Bar, Slider
+can be vertical or horizontal.
+
+Parts and Styles
+****************
+
+-  :cpp:enumerator:`LV_PART_MAIN` The background of the slider. Uses all the typical
+   background style properties. ``padding`` makes the indicator smaller
+   in the respective direction.
+-  :cpp:enumerator:`LV_PART_INDICATOR` The indicator that shows the current state of
+   the slider. Also uses all the typical background style properties.
+-  :cpp:enumerator:`LV_PART_KNOB` A rectangle (or circle) drawn at the current value.
+   Also uses all the typical background properties to describe the
+   knob(s). By default, the knob is square (with an optional corner
+   radius) with side length equal to the smaller side of the slider. The
+   knob can be made larger with the ``padding`` values. Padding values
+   can be asymmetric too.
+
+Usage
+*****
+
+Value and range
+---------------
+
+To set an initial value use :cpp:expr:`lv_slider_set_value(slider, new_value, LV_ANIM_ON/OFF)`. The
+animation time is set by the styles’ ``anim_time`` property.
+
+To specify the range (min, max values), :cpp:expr:`lv_slider_set_range(slider, min , max)` can be used.
+
+Modes
+-----
+
+The slider can be one of the following modes:
+
+- :cpp:enumerator:`LV_SLIDER_MODE_NORMAL` A normal slider as described above
+- :cpp:enumerator:`LV_SLIDER_SYMMETRICAL` Draw the indicator form the zero value to
+  current value. Requires negative minimum range and positive maximum range.
+- :cpp:enumerator:`LV_SLIDER_RANGE` Allows setting the start value too by
+  :cpp:expr:`lv_bar_set_start_value(bar, new_value, LV_ANIM_ON/OFF)`. The start
+  value has to be always smaller than the end value.
+
+The mode can be changed with :cpp:expr:`lv_slider_set_mode(slider, LV_SLIDER_MODE_...)`
+
+Knob-only mode
+--------------
+
+Normally, the slider can be adjusted either by dragging the knob, or by
+clicking on the slider bar. In the latter case the knob moves to the
+point clicked and slider value changes accordingly. In some cases it is
+desirable to set the slider to react on dragging the knob only. This
+feature is enabled by adding the :cpp:enumerator:`LV_OBJ_FLAG_ADV_HITTEST`:
+:cpp:expr:`lv_obj_add_flag(slider, LV_OBJ_FLAG_ADV_HITTEST)`.
+
+The extended click area (set by :cpp:expr:`lv_obj_set_ext_click_area(slider, value)`) increases to knob’s click area.
+
+Events
+******
+
+-  :cpp:enumerator:`LV_EVENT_VALUE_CHANGED` Sent while the slider is being dragged or
+   changed with keys. The event is sent continuously while the slider is
+   being dragged.
+-  :cpp:enumerator:`LV_EVENT_RELEASED` Sent when the slider has just been released.
+-  :cpp:enumerator:`LV_EVENT_DRAW_PART_BEGIN` and :cpp:enumerator:`LV_EVENT_DRAW_PART_END` are sent
+   for the following parts.
+
+   -  :cpp:enumerator:`LV_SLIDER_DRAW_PART_KNOB` The main (right) knob of the slider
+
+      -  ``part``: :cpp:enumerator:`LV_PART_KNOB`
+      -  ``draw_area``: area of the indicator
+      -  ``rect_dsc``
+      -  ``id``: 0
+
+   -  :cpp:enumerator:`LV_SLIDER_DRAW_PART_KNOB` The left knob of the slider
+
+      -  ``part``: :cpp:enumerator:`LV_PART_KNOB`
+      -  ``draw_area``: area of the indicator
+      -  ``rect_dsc``
+      -  ``id``: 1
+
+See the events of the `Bar </widgets/bar>`__ too.
+
+Learn more about :ref:`events`.
+
+Keys
+****
+
+-  ``LV_KEY_UP/RIGHT`` Increment the slider’s value by 1
+-  ``LV_KEY_DOWN/LEFT`` Decrement the slider’s value by 1
+
+Learn more about :ref:`indev_keys`.
+
+Example
+*******
+
+.. include:: ../examples/widgets/slider/index.rst
+
+API
+***