The manual is the only thing udpscope installs, but the only way to get it
where `help:` resolves was a whole `cmake --install`. Two opt-in targets
now install just the set, each printing where it wrote:
- install-help goes through the install machinery into CMAKE_INSTALL_PREFIX
(reachable via the new Help component), so a packager or a system-wide
user points it at /usr and wraps it in sudo;
- install-help-user resolves the user data directory the way the app does
($XDG_DATA_HOME, else $HOME/.local/share) and copies there without root.
Neither is built by default and neither changes what `cmake --install`
does; the install() rule keeps its destination.
