Document Rails installation

[jakduch]

Summary

• document the gems and initializer needed to run Dynflow in Rails
• explain in-process and separate production executor setups
• include Active Job, action, triggering, and common configuration examples
• link the new guide from the README

Testing

• smoke-tested the documented initializer with Rails 7.1, Active Record, SQLite, and a real Dynflow action execution

Fixes #421