Document intersection of Next.js major versions and Pantheon upstreams

[stevector]

Currently the creation of a new Next.js site involves specifying a Next.js major version.

terminus site:create my-site-name my-site-label nextjs15

However, the corresponding repository does not contain a Next.js version number in its name: https://github.com/pantheon-upstreams/nextjs

Given that Next.js 16 was released today, we will need to change something. I think my preference is to make a new repository under https://github.com/pantheon-upstreams/ for each Next.js major version. That is the pattern we follow for Drupal. (Though for Drupal site creation there is a complicated interaction between the major version repos and https://github.com/pantheon-upstreams/drupal-composer-managed that I don't fully understand)

Making a new repo/upstream for each Next.js major version will result in greater clarity for users and maintainers. If we had only one repo/upstream, I would expect that repo to always be on the latest major version. That would put extra pressure on us to time the switch from 15 to 16 (and 16 to 17) exactly right. That might be difficult if the release of a major version doesn't always mean we have the immediate confidence to make it the default option for all customers.