Docs/dashboard style updates

[rolfsmeds]

Dashboard docs updates for vaadin/web-components#8962

[github-actions]

AI Language Review

In the reviewed files:

1. dashboard-basic.ts: Ensure consistent use of custom CSS properties for widget styling. Currently, there is a mix of --vaadin-dashboard-item-rowspan and --vaadin-dashboard-widget-rowspan, as well as --vaadin-dashboard-item-colspan and --vaadin-dashboard-widget-colspan. They should be consistent across implementations for easier maintenance and clarity.

2. Styling.adoc:

   • One entry has a duplicate CSS property for "Minimum column width" with different default values. It should be corrected to avoid confusion.
   • Under style variants, "Flow DashboardVariant" should consistently reference the dashboard variant names or styles for clarity. Consider standardizing this section for better readability.

Please address these issues to improve consistency and clarity in the code and documentation.