[mailman3] guide cleanup

[noave]

some issues with mailman3 guide:

• [ ] installation should be done for production at first, so set Debug to False (there is even the problem, that hyperkitty wont work with debug True)
• [ ] mail to console debug should be deactivated as well
• [ ] add a debugging area, where is described how to enable debugging and where logs are found
• [ ] maildrop must be removed from .qmail-default, do not just add the mailman pipe
• [ ] we should consider using mysql db, not sqlite
• [ ] symlink or move all logs to ~/logs
• [ ] use current versions (dart-sass etc.)
• [ ] secretapikey should be explained as any random string
• [ ] make it less complex, just use the qmail-default, move the qmail-listname to a late feature section
• [ ] think about a feature to use spamfolder
• [ ] highlight code in "configure mailman core"
• [ ] we could remove the sqlite note etc