Add Sensu core/enterprise decom/removal instructions

[asachs01]

Description

We need to have proper decommissioning docs for Sensu core/enterprise. https://docs.sensu.io/sensu-core/latest/platforms/sensu-on-rhel-centos/#service-management doesn't really cut it.

Category

This is an issue with:

• [ ] errata (error in doc content)
• [x] new doc (requires a completely new doc)
• [ ] update (add missing or refresh existing content)
• [ ] improvement (improve docs content or format)