Add documentation to every function and automatic doc html building (#6)

Reviewed-on: Rubydragon/MetagraphOptimization.jl#6
Co-authored-by: Anton Reinhard <anton.reinhard@proton.me>
Co-committed-by: Anton Reinhard <anton.reinhard@proton.me>

src/diff/print.jl

@@ -1,3 +1,8 @@
+"""
+    show(io::IO, diff::Diff)
+
+Pretty-print a [`Diff`](@ref). Called via print, println and co.
+"""
 function show(io::IO, diff::Diff)
     print(io, "Nodes: ")
     print(io, length(diff.addedNodes) + length(diff.removedNodes))