Unify use of hyphens or underscores in file/directory names

Currently, some files and directories in the documentation use hyphens in their names (e.g. first-setup, guest-network or turris-10.jpg) and some of them use underscores (e.g. static_dhcp, foris_wan.md or mox_cpu_only.png). We should define what to use and then to unify it across the whole documentation.

I would recommend to use hyphens because:

1. Underscores can be overlooked easily.
2. Web search engines threats underscores as "not so separators". Thus names with hyphens are better indexed.