Add /docs/hardening to include hardening snippets by NwSEC

Added /docs/hardening as arranged w/ till to include snippets for

• WordPress Login AntiBruteforce (fully functional if the URL is /wp-login.php)
• postfix-ldap (needs valid, enterprise-specific LDAP data to be included)

All tested and productively implemented on Debian/GNU Linux by NwSEC, further documentation on https://blog.nwsec.de