Improve documentation

[parro-it]

Currently there is missing documentation about:

• [x] Area component
• [ ] Dialogs
• [ ] Grid component
• [ ] Add meaningful code example in VerticalBox, Horizontalbox, Group
• [ ] Menus
• [ ] Color
• [ ] move Point/Size from Area.md to a more central place
• [ ] in the readme:

Each libui widget implementation is written in it's own C++ file in src folder.

[mischnic]

#38 ?

[parro-it]

#38 closed