Use `.dia` extension instead of `.md`

[countvajhula]

Make any changes to scripts to expect this extension, and also change the existing DIA repos to use the new extension.

[benknoble]

Fortunately I don't think the Racket code has to change much at all; it's extensions agnostic AFAIK.

As we improve deanonymization and other project scripts (and even create a template??), we'll want to make changes there. But it really should be as simple as replacing the extension everywhere.