Update User Guide in README

[kevinlacaille]

The README’s link for the User Guide appears to be broken. The CLI Reference also appears to be out of date (V1).

Correct User Guide link: https://planet-sdk-for-python-v2.readthedocs.io/en/latest/
Correct CLI link: https://planet-sdk-for-python-v2.readthedocs.io/en/latest/cli/).