add instructions to create an emergency user

[1letter]

see https://community.plone.org/t/create-zope-emergency-user-with-wsgi-server/19796

[acsr]

The pull add notes to create an emergency user #1703 request misses a solution for docker based deployments created by Cookieplone projects and the resulting deployments:

• The purpose of an emergency user should be reflected in the docs: Create one when the system is inaccessible at runtime from outside
• Another scope would be how to add an additional ZMI admin in advance (actually not emergency but clever)

What is a better place to to enhance this: Seperate ticket? @stevepiercy @davisagli what do you think?