mxpy installer: warn users to pay attention to the output of the installer

[andreibancioiu]

Add a paragraph / note / warning to instruct users to pay attention to the output of mxpy-up:

• https://docs.multiversx.com/sdk-and-tools/sdk-py/installing-mxpy/

Related to: https://github.com/multiversx/mx-sdk-py-cli/issues/309.

[andreibancioiu]

Can be closed now, since mxpy is also installable using pipx (to be promoted even more after v10).