Add page akin to "leaving CLEX" for how to leave your org with shared data in good state

There is a page describing [what needs to be done before leaving your org on the CMS wiki](https://coecms.github.io/cms-wiki/starting/starting-leaving.html). However staff also leave other partner institutions leading to orphan data issues, it would be valuable to generalise that advice and add to this book's appendix or similar.