In a doc
Whether you're making a directory of content, or just need to quickly reference something, doc linking speeds up the process.
In your current doc, just start typing @ and the name of a doc to search (you'll also see recent docs listed by default):
In a comment
You can add doc links in your comments, as well.
Pasting URLs
If you paste another doc's URL into the body of the doc, that will update to show the doc's title, automatically.