Describe declarative installing of column generator for DataGrid

[sukhova]

Environment

• Documentation version: 7.2
• Part of the documentation: Manual
• Existing page: https://doc.cuba-platform.com/manual-7.2/gui_DataGrid.html#gui_DataGrid_addGeneratedColumn

Description of the bug or enhancement

Describe declarative installing of column generator for DataGrid according to cuba-platform/issues#1983