Add serialization support for `adjacency_matrix` and `compressed_sparse_row`

## Problem

As I noticed while updating the docs, currently only the adjacency list class has support for serialization: `adjacency_list` got serialization in `adj_list_serialize.hpp`

I expect the CSR to be the most straightforward to implement since it's "just" two arrays.

I have not found any complain online about this gap as being important or critical. Worth cataloging and looking into, though.