Document the support for 64-bit integer sparse matrix indices.

[hodgestar]

Describe the Issue!

By building QuTiP 5 with:

python -m build \
            --wheel \
            --config-setting="--global-option=--with-idxint-64"

one can use 64-bit integer indices.

Currently this is only documented in the changelog. We should also write a short guide on it, perhaps.

It might also be worthwhile adding a CI build for the 64-bit index case.

[DarkStarStrix]

I'd love to try to tackle this