TCF (Admin Guide): refactoring new chapters and divide terms 'roles' and 'users'

[maryiaLichko]

GH repo: https://github.com/tarantool/cluster-federation
Product: TCF
Since: 0.3.0 (Tech debt)
Root document: https://www.tarantool.io/en/clustersfederation/doc/latest/admin_guide/
SME: @ DifferentialOrange @ oleg-jukovec @ themilchenko
Examples:

Details

Customer request: rewrite chapters for better understanding and explain types of roles and users in Tarantool, TCF, etc.