Add documentation to setup the screen of the ergocub-head

[GiulioRomualdi]

This pull request includes updates to the documentation related to the ergocub-head setup process. The most important changes involve adding a new guide for setting up the ergoCub screen and updating the navigation to include this new guide.

Documentation updates:

• docs/icub_operating_systems/icubos/jetpack.md: Added a new link to the "Setup ergoCub screen" guide.
• docs/icub_operating_systems/icubos/setup-ergocub-screen.md: Created a comprehensive guide for setting up the ergoCub screen, including instructions for hiding the Ubuntu bar, changing the desktop wallpaper, and cleaning the icons from the desktop.

This is the outcome of https://github.com/ami-iit/lab-events-demos/issues/332#issuecomment-2438245878

cc @S-Dafarra

[pattacini]

Nice!
Just one inline suggestion I'll commit myself!

[pattacini]

Actually, the position in the documentation was wrong. Fixed via:

• 3c8a4e0