Add cuGraph examples

[ashvardanian]

We currently have a pretty extensive list of graph storage and processing approaches on the CPU, but lack GPU analogs. This can be a great opportunity to showcase both the more advanced and rarely used parts of the Nvidia libraries, as well as explore atomic variables and concurrent data-structure implementations on massively parallel chip architectures.

• cuGraph C++ docs
• PageRank C++ docs

This is also worth covering on the Python side with the available Python cuGraph bindings:

• PageRank example Jupyter notebook