Improve chart report documentation

[lovtsovaik]

Environment

• Documentation version: 7.2
• Part of the documentation: Reports
• Existing page: https://doc.cuba-platform.com/reporting-7.2/chart_report.html

Description of the bug or enhancement

See §3.5. Report with Chart Output

• in point 1: clarify that adding the Reports addon is also needed
• before point 2: login with admin/admin or give access to reports to another user (default user Joy have no access to reports)
• before point 3: add countable entity attributes manually because the mentioned project doesn't have its. If a project that has these attributes can be found, then replace the project link at the beginning.
• in point 3: clarify that the order of attributes is important to build the diagram correctly