Improve ngnix config (.well-known)

[martin-rueegg]

• avoid rule for /.well-known to apply also to e.g. /.well-known.xml
• deny access to /.well-known and /.well-known/
• provide "more understandable" explanation for "regex exception"
• make sure, /.well-known/acme-challenge and /.well-known/pki-validation are matched as a directory, not the beginning of some other service name
• use temporary redirect if nextcloud is not installed in webroot

☑️ Resolves

• Fix #…

🖼️ Screenshots

• /installation/nginx.html

[martin-rueegg]

Happy to backport, once accepted.