Add formulas to the documentation

[thepunitsingh]

Hi Team,

You are doing a great job with the igraph library. I am doing some analysis using python-igraph and getting issue understanding the functions in their mathematical terms. As we need to base our analysis on mathematical terms for research papers, please include the formulas of the functions in their documentation. Inclusion of formula has been beautifully done by Networkx, for example, please check one of their documentation page

Thanks.