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/graph/validate.jl

@@ -1,4 +1,8 @@
-# check whether the given graph is connected
+"""
+    is_connected(graph::DAG)
+
+Return whether the given graph is connected.
+"""
 function is_connected(graph::DAG)
     nodeQueue = Deque{Node}()
     push!(nodeQueue, get_exit_node(graph))
@@ -16,6 +20,11 @@ function is_connected(graph::DAG)
     return length(seenNodes) == length(graph.nodes)
 end
 
+"""
+    is_valid(graph::DAG)
+
+Validate the entire graph using asserts. Intended for testing with `@assert is_valid(graph)`.
+"""
 function is_valid(graph::DAG)
     for node in graph.nodes
         @assert is_valid(graph, node)