Diagrams documentation is missing

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

1. https://github.com/AlgebraicJulia/Catlab.jl/commit/8e69040f02f3db5554bda412ad36df60ec059937#diff-fd360ef0e1aa3cd9fd0d8997d151dfca7160e6e3eeafffb049aacf1a11336448L19
2. https://github.com/AlgebraicJulia/Catlab.jl/commit/5438653c1f79d187075ddf55228227953f75620e

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](https://github.com/AlgebraicJulia/DataMigrations.jl) are currently [the template docs](https://algebraicjulia.github.io/DataMigrations.jl/dev/) and the [stable](https://algebraicjulia.github.io/DataMigrations.jl/stable) branch is a 404.

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