Update package install instructions for packages on PyPI

Several DESI packages are now available on PyPI ([desiutil](https://pypi.org/project/desiutil/), [desitarget](https://pypi.org/project/desitarget/), [desispec](https://pypi.org/project/desispec/), [redrock](https://pypi.org/project/redrock/), etc.), which presents the opportunity to simplify software installation instructions.