[Config] Add a note about a new configuration approach to each topic that uses box.cfg

[andreyaksenov]

Before the 3.0 release, we need to add a note about a new configuration approach at the top of each topic that uses box.cfg for configuring instances. These notes can be removed after updating each topic to a new config.

Affected topics:

• https://docs.d.tarantool.io/en/doc/3.0/enterprise/security/
• https://docs.d.tarantool.io/en/doc/3.0/enterprise/audit/
• and so on (see [Epic] 3.0 config - Part 1 #3544)