Improve docs covering self-signed certificate management / internal CA usage

[joshtrichards]

• https://docs.nextcloud.com/server/latest/admin_manual/configuration_files/external_storage_configuration_gui.html#using-self-signed-certificates
  • Links to a 3rd party blog post from 2014 that describes an approach that doesn't even exist in our current code base
• https://docs.nextcloud.com/server/latest/admin_manual/configuration_server/occ_command.html#security
  • Lists the commands, but is the only place that does so... kind of buried
• https://docs.nextcloud.com/server/latest/admin_manual/configuration_files/external_storage/ftp.html#ftp-ftps
  • vaguely states "additional configuration" will be necessary

Triggered by this thread: https://help.nextcloud.com/t/installation-issue-cmd-curl-is-successful-but-fails-from-gui-ssl-issue-connecting-to-openproject-url/194990/4