Consider integrating igraph.r2cdocs here

The logic is in a single file, it would simplify processes as we work on integrating v1.0.x .

In DESCRIPTION we declare the use of the docs_rd() roclet. That roclet, if defined within igraph, doesn't exist at the beginning of the document() workflow, because roxygen2 first gets the roclets then loads the package.

So integrating igraph.r2cdocs might prove problematic. We could load igraph before running document().