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

docs/widgets/msgbox.rst

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+Message box (lv_msgbox)
+=======================
+
+Overview
+********
+
+The Message boxes act as pop-ups. They are built from a background
+container, a title, an optional close button, a text and optional
+buttons.
+
+The text will be broken into multiple lines automatically and the height
+will be set automatically to include the text and the buttons.
+
+The message box can be modal (blocking clicks on the rest of the screen)
+or not modal.
+
+Parts and Styles
+****************
+
+The message box is built from other widgets, so you can check these
+widgets’ documentation for details.
+
+- Background: `lv_obj </widgets/obj>`__
+- Close button: `lv_btn </widgets/btn>`__
+- Title and text: `lv_label </widgets/label>`__
+- Buttons: `lv_btnmatrix </widgets/btnmatrix>`__
+
+Usage
+*****
+
+Create a message box
+--------------------
+
+:cpp:expr:`lv_msgbox_create(parent, title, txt, btn_txts[], add_close_btn)`
+creates a message box.
+
+If ``parent`` is ``NULL`` the message box will be modal. ``title`` and
+``txt`` are strings for the title and the text. ``btn_txts[]`` is an
+array with the buttons’ text. E.g.
+``const char * btn_txts[] = {"Ok", "Cancel", NULL}``. ``add_close_btn``
+can be ``true`` or ``false`` to add/don’t add a close button.
+
+Get the parts
+-------------
+
+The building blocks of the message box can be obtained using the
+following functions:
+
+.. code:: c
+
+   lv_obj_t * lv_msgbox_get_title(lv_obj_t * mbox);
+   lv_obj_t * lv_msgbox_get_close_btn(lv_obj_t * mbox);
+   lv_obj_t * lv_msgbox_get_text(lv_obj_t * mbox);
+   lv_obj_t * lv_msgbox_get_btns(lv_obj_t * mbox);
+
+Close the message box
+---------------------
+
+:cpp:expr:`lv_msgbox_close(msgbox)` closes (deletes) the message box.
+
+Events
+******
+
+-  :cpp:enumerator:`LV_EVENT_VALUE_CHANGED` is sent by the buttons if one of them is
+   clicked. :cpp:enumerator:`LV_OBJ_FLAG_EVENT_BUBBLE` is enabled on the buttons so
+   you can add events to the message box itself. In the event handler,
+   :cpp:expr:`lv_event_get_target(e)` will return the button matrix and
+   :cpp:expr:`lv_event_get_current_target(e)` will return the message box.
+   :cpp:expr:`lv_msgbox_get_active_btn(msgbox)` and
+   :cpp:expr:`lv_msgbox_get_active_btn_text(msgbox)` can be used to get the
+   index and text of the clicked button.
+
+Learn more about :ref:`events`.
+
+Keys
+****
+
+Keys have effect on the close button and button matrix. You can add them
+manually to a group if required.
+
+Learn more about :ref:`indev_keys`.
+
+Example
+*******
+
+.. include:: ../examples/widgets/msgbox/index.rst
+
+API
+***