sphinx: update install documentation, mimic README
sphinx: update install documentation, mimic README

Update the Sphinx `install` chapter, making it closer to the main
`README.md` instructions. The steps related to installing the SDK
via `git clone` are removed, assuming that the document is targeted
towards end users.

QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py