docs: document addIcons usage

Some users have expressed an interest in using functionality provided by addIcons, so it would be good to document its usage.

Use Case: https://github.com/ionic-team/ionic-framework/issues/22181 A developer wants to dynamically show an icon based upon results from a server. They could do `(result) ? heart : heartOutline`, but they do not want to have to continually import each icon. In this case, it would be helpful for them to add the icon globally via `addIcons`.