documentation

Pull request #1376: feature/ZBXNEXT-8145-7.0-D: Communication framework for widgets.

Merge in WEB/documentation from feature/ZBXNEXT-8145-7.0-D to release/7.0

* commit '3bb387c0186bbecd77f3d3dfc39bf2efae1c255a':
  [ZBXNEXT-8145] Communication framework for widgets (refined wording)
  [ZBXNEXT-8145] Moved section on Time period selector
  [ZBXNEXT-8145] Communication framework for widgets