Diagrams documentation is missing

[jpfairbanks]

The documentation for computing with diagrams in Catlab is gone, I have tracked down a few commits that removed it.

1. 8e69040#diff-fd360ef0e1aa3cd9fd0d8997d151dfca7160e6e3eeafffb049aacf1a11336448L19
2. 5438653

I think that these were intending to relocate the material into DataMigrations.jl docs, but those aren't building correctly.

The docs at DataMigrations.jl are currently the template docs and the stable branch is a 404.

@KevinDCarlson, can you make sure this gets fixed ASAP?