[Config] Document 'app' configuration settings

[andreyaksenov]

Related doc issue: #3702

Product: Tarantool
Since: 3.0
Root document:

• A new app section in the Configuration reference.
• https://www.tarantool.io/en/doc/latest/concepts/configuration/#loading-an-application
• Mention roles

SME: @ Totktonada

Details

Options:

• https://github.com/tarantool/tarantool/blob/f58bfc97c627f1838696697339fb3e443c05767a/src/box/lua/config/instance_config.lua#L1630

[andreyaksenov]

Reference: https://www.tarantool.io/en/doc/latest/reference/configuration/configuration_reference/#app