add vignettes

[schochastics]

For the ISC Project, we promised to add vignettes. This is a running list of topics (in no particular order) that definitely need a vignette (feel free to add to it)

• centrality https://github.com/igraph/rigraph/tree/vignette-centrality
• clustering / community detection https://github.com/igraph/rigraph/tree/vignette-clustering
• basic network descriptives (density, transitivity, etc) https://github.com/igraph/rigraph/tree/vignette-descriptives
• plotting
• (random) network generators https://github.com/igraph/rigraph/tree/vignette-graph_gen
• ...

[maelle]

weights?

[schochastics]

weights should be addressed in each vignette. I think it is better to do this topicwise, not a specific weights vignette