Add code example about Conditional Formating in Spreadsheet

The chapter about Conditional Formatting has only a text.

https://vaadin.com/docs/latest/components/spreadsheet#conditional-formatting

This is not enough. One needs to use ConditionalFormattingRule from POI, etc. in order to for example achieve value dependent color setting. A basic code example about e.g. positive / green, negative / red, would be helpful.
