Revamp the visuals and documentation of Simple Scatter Graph example

Simple Scatter Graph example is the entry point example for Qt Data Visualization, so we will keep it mostly as-is.

Some visual updates can be done, as well as checking that everything mentioned in the docs is still valid with cmake build system too.

https://intranet.qt.io/pages/viewpage.action?pageId=161517211#ExamplesforH1/23bonustarget-QtDataVisualization