[cryptpad] improve the guide security

[noave]

Cryptpad serves a checkup page that returns some errors with installing at isabell.uber.space/checkup

• at first we should follow the "two-domains" directive and advise to setup a domain unsafe.isabell.uber.space as UnsafeOrigin
• most errors refer to missing headers, there is an example nginx config which should be used to set up headers like stated in the manual https://manual.uberspace.de/web-headers/
• it looks like in newest version 4.14 there is no longer an email configuration within the config file, check if this should still be added

review: https://docs.cryptpad.fr/en/admin_guide/installation.html#domains

[noave]

looks like we need to set

• //httpSafeOrigin: to something like safe.isabell.uber.space
• //httpUnsafeSafeOrigin: to isabell.uber.space

and we need to tell how to create an admin account