More Robust 3.x to 4.x documentation

https://codeigniter4.github.io/CodeIgniter4/ is not the official User Guide.
It is the current in-progress User Guide.
If you will see it as the official one when the next version is released.

> If there are things missing, should I post them in this thread or create an issue in Github?

GitHub issue is better. It will definitely catch the attention of the maintainers.
But either will do.

ci-phpunit-test | CodeIgniter Testing Guide | CodeIgniter 3 to 4 Upgrade Helper