introduce 'util' category of plugins

* this will help to make master.cfg files that only use core functinality to
  look much simpler
update sample for master.cfg to use plugins
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