documentation

Pull request #745: feature/ZBXNEXT-7836-6.4-D (Make maintenance configuration form modal).

Merge in WEB/documentation from feature/ZBXNEXT-7836-6.4-D to release/6.4

* commit '8558ee1b1090cb56df0987e96c146c51bfcb2208':
  Combined entries about multiple forms converted into modal (pop-up) windows.
  [ZBXNEXT-7836] Make maintenance configuration form modal (updated pages "What's new", "Maintenance")