Add a copyable snippet to the Deploy with Pulumi button page

[cnunciato]

It'd be much easier to drop these into READMEs and things if we had an already-formatted Markdown snippet to copy on this page. (Right now you have to figure this out for yourself by right-clicking the button, finding out what the image URL is, and creating a Markdown image link by hand, which even I mess up about half the time.) We should fix this.