There's no functionality for doing this in rich text editors in Wagtail at the moment. You could either not use rich text and define the links as properties on the page, have the editors simply type out the links, or if you're willing to get your hands dirty you can build a custom Wagtail using Tim Heap's
Unify link choosers pull request. You would then define a new link chooser that inherits from wagtail.wagtailadmin.link_choosers.LinkChooser, register it with @hooks.register('register_rich_text_link_chooser') and define some views and templates for the new chooser. Note that custom link choosers are not officially supported so if you go down that route you'll be heading into unknown territory.