Documentation: Vital info only in 'Upgrading to tox v4'

[Reclocco]

Issue

Some vital information about Packaging Environments and it's config can be found only in 'Upgrading to tox v4' documentation section. I'm not against it being there but it should also be included in regular sections like 'Configuration'. I think there are also other bits of general/fundamental information found only in the 'upgrading' section.

[jugmac00]

Thanks for raising this!

As version 4 is the only one we maintain, your suggestion makes a lot of sense.

Help and contributions would be very welcome.