Improve documentation

[gdalle]

Following this Discourse thread, it would be nice to document some essential methods of the package, such as weight(e::SimpleWeightedEdge)
cc @etiennedeg

[gdalle]

While we're at it, we may want to use CI and switch to a Documenter-based model, see #21

[gdalle]

See also #16

[gdalle]

Solved by #38