Don't recommend virtualenv for devstack

[jmbowman]

While reproducing the steps that tripped up a new devstack user, I noticed that the Getting Started guide says to set up a Python virtualenv, but most users won't need a virtualenv installed for normal devstack usage. Only the following tasks would still need a local Python virtualenv:

• Building documentation locally (but this can be deferred to CI and RTD)
• Updating dependencies (but we have a GHA workflow for this)
• Running tests locally (but these are for code most developers never touch)

Especially now that docker-compose has been rolled into Docker, we just shouldn't need to install any Python packages outside of Docker for normal devstack usage.

A/C:

• Verify that most devstack functionality works without a virtualenv
• Update the Getting Started guide to no longer ask new users to set up a virtualenv
• Update any other docs for the handful of tasks still requiring a virtualenv to mention the need and steps for setting one up
• Ensure that the instructions accurately inform Linux users how to get a working docker-compose without using a virtualenv

[timmc-edx]

It looks like on current Ubuntu LTS there's a docker-compose package that's up to date, whereas the docker.io package (version 20.10.21) does not yet support the docker compose subcommand. So I think we'll need to stick with docker-compose for now.