[DOC] describe mxgraph available styles for sketch settings

[tbouffard]

Same as draw.io, which is a bridge to roughjs configuration properties.

• roughjs doc: https://github.com/rough-stuff/rough/wiki#options
• available styles:

  mxgraph-sketch/src/SketchySvgCanvas.ts

  Line 78 in 4856acc

  // Applies cell style