Add tiers to the glossary and CLI

Glossary

• Add tier definition to the glossary
• Edit cluster definition accordingly
• Remove instance groups
• Edit replication factor

CLI

• Add picodata run --init-cfg
• Add picodata run --tier
• Edit picodata run --init-replication-factor

Part of #101 (closed)

See also picodata#330 (closed)

Follow-up for picodata!695 (merged)