Not seeing anything related to these in the Metadata types documenation.
Can anyone give me an example of how these can be referenced?
They are in the objects metadata with the webLinks tag.
Sure in my Account metadata I have the following link:
The correct way to reference these in the Ant Migration tool is to create a WebLink metadata type in your package.xml file. The actual detail of the button/link will be stored within the custom object xml file. For example:
Will result with the metadata of the Send_SMS button inside of the Contact metadata file in the objects folder:
The button can then be retrieved and deployed as desired.