hub_score+authority_score man page

[maelle]

by @szhorvat in #660

This is probably not a subject of this PR, but hub_score() and authority_score() should share a single documentation page. Future versions will have a single function to compute both of these at the same time, in a way that the result match.

[szhorvat]

FYI separate versions of these functions was just deprecated in the C core: igraph/igraph#2320