Welcome to magnus talk your trusty dialogue plugin
To download go to this link.
Unzip the newly dowloaded file.
Paste the addons folder from the unzipped file into you godot project.
Go into: project settings > plugins.
Enable the MagnusTalk plugin.
If you see a Magnus Talk popup then the plugin has been installed, if you don't restart the editor.
You can either use the documentation built in with the plugin or you can use the documentation here.