Update docs: yadm is available on main Arch repo now

[bound-variable]

This issue is about

• [ ] Man pages or command-line usage
• [x] Website documentation
• [ ] Packaging
• [ ] Other

Describe the issue

The docs state that yadm can be installed on Arch through the AUR, but the program is now available on the main repo.

[TheLocehiliosan]

This has been merged into the dev-pages branch. It will be published soon when the website is updated for the new release.