documentation

Pull request #1884: feature/ZBXNEXT-9215-7.0-D. Select widgets as data source for items in Pie chart and Graph widgets.

Merge in WEB/documentation from feature/ZBXNEXT-9215-7.0-D to release/7.0

* commit 'c321f905f1b1994b893c6ca2e6bd9deab1f1bb90':
  [ZBXNEXT-9215] removed article
  [ZBXNEXT-9215] fixed shortcomings
  [ZBXNEXT-9215] select widgets as data source for items in Pie chart and Graph widgets
  [ZBXNEXT-9215] added "Data set" section introduction
  [ZBXNEXT-9215] refined "Data set" section introduction
  [ZBXNEXT-9215] improved "Data set configuration details" section
  [ZBXNEXT-9215] moved "Data set configuration details" closer to data sets description
  [ZBXNEXT-9215] reworked Graph widget Data set description (based on Pie chart widget Data set description)
  [ZBXNEXT-9215] updated wording
  [ZBXNEXT-9215] select widgets as data source for items in Pie chart and Graph widgets