fix: Steam deck installation and .desktop fix

[Nannk]

Pull Request Template

Type of change

• [x] Bug fix
• [ ] Feature
• [x] Documentation update

Description

Makes ani-cli installation instructions for steamdeck work. Also enables easy launch from Steam "game mode" Fix for this https://github.com/pystardust/ani-cli/issues/1268

Checklist

• [x] any anime playing

• [ ] bumped version

• [ ] next, prev and replay work
• [ ] -c history and continue work
• [ ] -d downloads work
• [ ] -s syncplay works
• [ ] -q quality works
• [ ] -v vlc works
• [ ] -e select episode works
• [ ] -S select index works
• [ ] -r range selection works
• [ ] --dub and regular (sub) mode both work

• [ ] all providers return links (not necessarily on a single anime, use debug mode to confirm)

• [ ] -h help info is up to date
• [x] Readme is up to date
• [ ] Man page is up to date| i guess?