update Arch install guide to use stable packages

Encourage the use of the stable packages per disro norms. The devel
packages can be used but they trigger full git clones in many AUR
helpers and are generally a pain if you don't specifically want them for
something. Mention the devel package but update the URLs to point to
the one that uses the normal packaging system.
refactor install instructions to include myrepos

Since myrepos is so strongly recommended, included it in the distro
specific install instructions. Refactor the Arch Linux instructions
to better reflect most common usage.