Improve the docs for sparse matrices

It is almost impossible to learn how to use SparseMatrix from the [current documentation](https://mathjs.org/docs/reference/classes/sparsematrix.html). The [example code](https://mathjs.org/examples/sparse_matrices.js.html) also isn't very useful. We need to improve the documentation and make it easier to grasp and include some real-world examples.

(Possibly related to #1166 and #1770?)