Add links to Roles & Access Groups videos

[alexbudarov]

Environment

• Existing page: https://doc.cuba-platform.com/manual-7.2/roles.html#roles_run_time
  https://doc.cuba-platform.com/manual-7.2/groups.html

Description of the bug or enhancement

Similar to #663

Feedback from Haulmont newbie:

Not enough examples for me in the section how to set up security.

(he means runtime security, I think)

Solution:
Add links to YouTube videos as examples.
Roles section: https://www.youtube.com/watch?v=LT4yu_2ocMY
Access groups section: https://www.youtube.com/watch?v=48tM4jTl0tc

[knstvk]

Unfortunately, roles video is outdated.

[alexbudarov]

ah, deny by default (
yes, roles video needs re-filming