Cleanup installation docs

[traversaro]

In particular:

• Specify that the installation documentation is for Ubuntu/Debian system
• Do not install python3.8 explicitly, that would fail on Ubuntu 22.04, instead just rely on the fact that python3.* is a dependency of python3-pip and python3-venv
• Avoid to suggest users to install in a global location. These docs are tipically followed blindly by people with not a lot of experience, and if we suggest to install in a globally visible location (be it /usr/local or ~/.local) there is an good chance that something will either do not work in mann-pytorch installation, or that mann-pytorch will break some other workflow. A good alternative is to always suggest to install in a virtual environment. Naive users will execute everything without interfering with the rest of the setup, and expert users that for any reason do not want to install in a venv they will be able to do that if they want.

[paolo-viceconte]

Thanks for the suggestions! I also doublechecked the updated installation procedure and it works fine on Ubuntu 20.04.