documentation

Pull request #1821: feature/ZBXNEXT-8316-7.0-D. Added Item history widget.

Merge in WEB/documentation from feature/ZBXNEXT-8316-7.0-D to release/7.0

* commit '0f38a483cc79cd6146f0930c9863b3a22b393071':
  [ZBXNEXT-8316] fixed shortcomings
  [ZBXNEXT-8316] updated image size
  [ZBXNEXT-8316] added "Item history" widget images; updated image sizes
  [ZBXNEXT-8316] clarified "Highlights" parameter description
  [ZBXNEXT-8316] added "What's new" & "Upgrade notes" entries
  [ZBXNEXT-8316] clarified "Display" parameter description
  [ZBXNEXT-8316, ZBXNEXT-9193, ZBX-24488] added Item history widget