Improve installation steps

[stsewd]

This is based on the recent questions about setting up the project.

Probably we may want to check this other PR #3498 too

[ericholscher]

I think adding more caveats to our install instructions is not the answer. Our install instructions are already too complex, and have too many caveats. I don't know what the answer is, but teaching people how to use virtualenv's in our install docs seems like something we shouldn't be doing.

[stsewd]

@ericholscher yeah, you are right, the docs already provide a link to the virtualenv pip page, perhaps would be better a link to the user guide https://virtualenv.pypa.io/en/stable/userguide/? Also notice that the firsts caveats are just requirements, probably grouping them in a section would be better.

Closing for now.