Update doc wrt. accessing FTP servers on the local network

Accessing FTP servers on LAN changed in Nautilus, this affects the section “Downloading files from an FTP server on the local network” on doc/advanced_topics/lan.mdwn.

Feature Branch: doc/12376-lan-ftp

Parent Task: #11916 (closed)

Related issues

Blocks #12432 (closed)

Original created by @spriver on 12376 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information