Add samples of symbol rendering

[DelazJ]

To complete #1737 it could be nice to "decorate" the Configure a symbol section with some sample of symbols rendering. It'd visually help readers to get what each symbol layer type can look like (or kind of things they can do when combining them). I'm currently thinking of:

• [ ] the geometry generator
• [ ] the arrow symbol (as line symbol or polygon boundary)
• [ ] the gradient fill symbol
• [ ] the shapeburst fill symbol
• [ ] the filled marker symbol
• [ ] the vector field marker symbol

Opinions? Someone willing to take the lead?

[malleeblue]

I am willing to have a go at this.

[DelazJ]

Thanks ans welcome @malleechick

[timlinux]

@DelazJ and @selmaVH1 what do you think about us just linking over to the cheat sheet that Kartoza publishes? https://kartoza.com/qgis-resources?