Document onboarding process for new maintainers

[jahrlin]

Our current process for onboarding new maintainers to cert-manager is quite manual and involves many steps, making it error prone.

We need to document the steps that need to be taken by the other maintainers, as well as the new member, such as:

• adding the new user to relevant Google groups
• adding the new user to OWNERS
• submitting an issue to add the user to CNCF maintainer index
• add the user to the correct GitHub teams