docs: improve install + troubleshooting README

[ulises-jeremias]

Problem

New Arch users need yay/paru commands and version-sync notes vs GitHub Releases.

Proposed solution

Expand README; link Discord and website install.sh as alternative.

[ulises-jeremias]

Taking this one myself — Homebrew/AUR packaging work needs org-specific permissions (tap publish, AUR credentials, distribution secrets), so it is not a good fit for external contributors right now.

If you want to help with related docs/tests that do not require those secrets, ping me in Discord and we can carve out a safe slice.