GraphsFlows.jl is unreachable from docs

[juliohm]

The current documentation doesn't even mention the existence of GraphsFlows.jl. It deserves a "Flows" section in "Algorithms API". This might also be true for other modules that are not shipped with Graphs.jl.

[Krastanov]

Really, we need generally a more unified way to document GraphsFlows, GraphsColoring, GraphsMatching, and GraphsOptim.