Ask HN: What are the “View issue” and “Modify reservation” buttons in Gmail?

This morning, my Gmail inbox had a message with a small gray button on the right-hand side that said "Modify reservation". It was an email from my son's baseball team. I've seen similar "View issue" buttons that point to Github issues.

How does Gmail do this? There must be some standard content in the body of the email, some kind of semantic markup, that tells Gmail what the link means. But I can't find documentation on it anywhere.

The ability to put a button in someone's inbox could be a really useful feature for a variety of apps. Where are the docs?