Update install guide to include Arch packages

These are official parts of the community repo, and this removes a 'FIXME'.

The sequoia "group" is emphasized over the individual package. It contains not only sq, but also sqv and the Python & C bindings.

If the user doesn't read too many details and just copies the pacman command, this will give them spv along with sq. This is probably what most users want by default (the bindings are pretty handy too).

If the user wants to install the individual sequoia-sq package, that is available in the next paragraph. However this comes with a note stating that installing it will intentionally avoid spv and the C bindings (which may be what the user wants).

I have included a note about Arch being rolling release in order to contrast with the warning on Debian. Users may not be aware of the differences and I don't want them to think the Arch packages are outdated (in practice they rarely are).