Describe in greater detail the handling of the engines property in package.json

[stevector]

Our Next.js overview page says

Pantheon provides the latest 3 LTS versions of Node.js. Pantheon derives which one to use by looking at the engines property in package.json.

We should document in greater detail what happens if

• a site tries to use an older version (Node.js 14)
• a site tries to use an non-LTS version
• a site tries to specific any given exact patch version (There are many releases of v24 https://nodejs.org/en/download/archive/v24.11.1. Could someone get v24.0.1 by specifying that exact version or do they get 24.11.1 because it is the latest?)