docs: guides for subscription tiers and token mgmt

This PR introduces two new user-facing guides:

- **Subscription Tiers**: explains concepts, architecture, tier-to-group
  mappings, RBAC design, rate limiting and troubleshooting.

- **Token Management**: describes how users obtain, use, and revoke
  service account tokens, including lifecycle management.

Both guides include diagrams, examples, and verification steps to make
setup and usage more approachable.

Signed-off-by: Bartosz Majsak <bartosz.majsak@gmail.com>

Author: bartoszmajsak