How do I add fields to the page properties dialog?
/libs/foundation/components/page/dialognode to your template's component that you want to add dialog properties to.
If you had a page component
/apps/<myapp>/components/pages/contentpage that has
sling:resourceSuperType=/libs/foundation/components/page then you would copy
/apps/<myapp>/components/pages/contentpage/ to make an overlaid dialog