Rewrite a few pages

https://www.yiiframework.com/contribute
https://www.yiiframework.com/report-issue

Here are only pages about Yii 2 or Yii 1 and there no words about Yii 3.
Need to add examples how to contribute and report issues for Yii 3. But we have one problem – Yii 3 doesn't have a "main" repo to report issues :)
Maybe an interactive page with selector of pages solve this problem? 