Add how to set Chart series colors to "CSS Styling" docs page

[Haprog]

https://github.com/vaadin/vaadin-charts-flow/blob/master/documentation/java-api/css-styling.asciidoc this page should have info about how to set the chart colors using Java API (so that it is findable via https://vaadin.com/docs/v14/charts/java-api/css-styling.html).

The colors can be set via CSS custom properties --vaadin-charts-color-{n} but afaik those are currently documented only here: https://vaadin.com/components/vaadin-charts/html-api/elements/Vaadin.ChartElement (see "Setting colors").

Maybe we could even link to the above HTML API docs page for more info but it should be pretty straight forward to at least show how to set those CSS variables with the Java API for low hanging improvement.

[jhult]

I believe the primary method to set colors was changed as part of vaadin/vaadin-charts-flow#351.

Try using setColor on a class implementing AbstractSeriesItem.

[web-padawan]

Note, the corresponding CSS styling documentation for Charts is now located here.

[web-padawan]

Moved the issue from the web component repository as it makes more sense to keep it here.