make formatting related changes to the affected documents

* this will help the rebase should they have changed in the base branch
* do some whitespace cleanup (trailing whitespaces, empty lines)
* move change hooks to the end of customization.rst (relies on
  non-existent feature, hence it's not clear how to use it)
* a bit of extra formatting (not part of this change, but anyway)
update user documentation
update release notes