Documentation, suggest to include default theme examples

A suggestion to add the default theme button class example.

All users will have access to these themes and are more likely to be testing plugins with the default themes. They also work by default in the front end, backend as well as when editing content.

It is also suggested that all extra themes and plugins are turned off when bug reporting.

Example configuration for Default Themes - Olivero and Claro

a.button|Button (Olivero/Claro)
a.button--primary|Primary (Olivero/Claro)
a.button--small|Small (Olivero/Claro)
a.button--extrasmall|Extra Small (Claro)

These are the only examples I spotted checking the following files:

• core/themes/olivero/css/components/button.css
• core/themes/claro/css/components/button.css