[Epic] 3.0 release documentation

[veod32]

Create and publish 3.0 release documentation.

Scope

• 3.0 release notes #3517
• [Config] Add a note about a new configuration approach to each topic that uses box.cfg #3913
• remove an empty guide: Migrating to declarative configuration
• Cleanup empty pages in EE doc section (dev guide, cluster admin guide)

Website and CI tasks

• Update CI with 3.0 release #3955
• tarantool/tarantool.io#1488