Add more detail to Yii 2 -> Yii 3 migration guide

[samdark]

See https://danaluther.blogspot.com/2021/03/converting-application-from-yii2-to-yii3.html

[samdark]

• How to convert a controller from Yii 1.1 to Yii 3
• What’s needed to port ActiveRecord classes from 1 to 3
• What changed in the rendering engine/view files
• What changed in the configuration files

See https://forum.yiiframework.com/t/knowledge-base-for-yii-1-to-3-migration/132672/2 as well. We want 1.1 -> 3.0 guide as well.

[olleharstedt]

Also some info about the gridview widget, perhaps. We use that a lot.

[olleharstedt]

A big one for ActiveRecord porting: Both in Yii 1 and 2, validation is part of ActiveRecord class - in Yii 3 it's factored out. A small example model and how it will be changed in Yii 3 would be nice.