docs for operator key generation / public key enrollment

[evq]

this was improved in https://github.com/brave-intl/bat-go/pull/2131, but it could use docs

[husobee]

• Operator will run ssh-keygen to generate a new operator key
• Operator will change services/payments/authorizors.go to include newly generated key
• Operator will seek review and approval from a codeowner of said file.

[husobee]

Updating Keys? Removing Keys? Changing Keys?

What are the policies for these actions.