Improve documentation (#1058)

* Remove @ingroup from class member definitions
* Add the documentation for every public classes.

include/ftxui/dom/canvas.hpp

@@ -20,6 +20,21 @@
 
 namespace ftxui {
 
+/// @brief Canvas is a drawable buffer associated with drawing operations.
+///
+/// Canvas is a drawable area that can be used to create complex graphics. It
+/// supports drawing points, lines, circles, ellipses, text, and images using
+/// braille, block, or normal characters.
+///
+/// Note: A terminal contains cells. A cells is a unit of:
+/// - 2x4 braille characters (1x1 pixel)
+/// - 2x2 block characters (2x2 pixels)
+/// - 2x4 normal characters (2x4 pixels)
+/// 
+/// You need to multiply the x coordinate by 2 and the y coordinate by 4 to
+/// get the correct position in the terminal.
+///
+/// @ingroup dom
 struct Canvas {
  public:
   Canvas() = default;