Check out the apostrophe-rich-text-permalinks module!
Users can click the usual “link” icon in the rich text editor, then pick “Document” from the “Link Types” dropdown menu.
Now they can click the “Browse Documents” button and select the page they wish to link to.
The resulting link will always stay up to date, even if the page moves around the site and its slug changes. This is the main advantage, in addition to the simple convenience of picking a page via the page tree.
You can also configure it to allow selecting particular piece types as well as pages. Anything that will have a _url property is fair game.