INSTALL.md: point Ubuntu users at the ppa:rsyncproject/rsync PPA

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Most Ubuntu users landing on INSTALL.md want to install rsync, not
build it.  Add a short section near the top that offers the
Launchpad PPA as the one-line path for the four currently supported
series (jammy 22.04 LTS, noble 24.04 LTS, questing 25.10,
resolute 26.04 LTS), and clarify that the rest of the file is about
building from source.

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