doc updates on remote-access

Hello! I made some changes to the remote access folder for ftp and ssh. 

FTP: Basically I suggested not to use it, but use SCP and SFTP instead

SSH: Added a couple things in the ssh-keygen section, and suggested the use of `ssh-copy-id` instead of the other method. 

Unix: Also added an example `nmap` command and suggest a very useful network scanner `fing`.