Add guidelines on how to setup and use virtual environments

[jorgepiloto]

Virtual environments are a fundamental concept and tool in the Python ecosystem. They allow developers to have an isolated development environment and avoid dependencies conflicts when working across several projects.

This could be added within the Development practices section and should include the following subsections:

• Why virtual envs exist/matter.
• How to setup a fresh and clean venv.
• Hot to check, activate and deactivate those.
• Editable installs?

[jorgepiloto]

It would be convenient to explain how tools like flit and poetry interface with venvs. Poetry will automatically create a venv in a default location depending on the OS being used and activate a custom venv for development purposes, see official poetry docs.

[jorgepiloto]

Closing this in favor of #64. This must be implemented under how-to/setup/venv.rst.