Ensure the Content Pipeline extension limitation to .NET 8 libraries is very clear

[SimonDarksideJ]

Summary

As the MGCB editor is a DotNet 8 tool, there is a hard limitation on content pipeline extensions also needing to be no higher that DotNet 8.

As the framework has evolved, the documentation has not kept up to date to make this very clear and docs should be reviewed and updated with an important note to document the limitation.