Documentation for custom icons in 1.1

[jtpio]

There doesn't seem to be any documentation yet on how to add custom icons from third-party extensions, using the new iconRegistry introduced in 1.1.

import iconSvg from '../style/icon.svg';
defaultIconRegistry.addIcon({name: 'icon', svg: iconSvg})

Upgrading from 1.0 to 1.1 also seems to be breaking toolbar icons (they are not shown anymore) when third-party extensions define their icons directly in CSS with background-image.

It would be great to add a few points to the docs explaining how to:

• add custom icons to the registry
• add them to toolbars
• handle themes

[jtpio]

Closing as there is now documentation about icons:

• https://jupyterlab.readthedocs.io/en/stable/developer/ui_components.html#ui-components
• https://github.com/jupyterlab/jupyterlab/blob/master/packages/ui-components/README.md

Fixed by #7767 for 2.0.