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

• 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

Start with the self-signed certificates section in external_storage_configuration_gui.html, the Security section of occ_command.html, and the FTP/FTPS section in external_storage/ftp.html. Compare their guidance with the current code and configuration, then update the outdated or buried information and explain the required additional configuration for internal CAs. Done means the three pages provide current, discoverable, consistent guidance without relying on the obsolete third-party post.