Check Social Login section and upgrade sample-social-login project

[knstvk]

Environment

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

Description of the bug or enhancement

The sample project is now on CUBA 6.8. We should upgrade it to 7.2 and check the doc.