build: install the manual alone, into the prefix or the user datadir

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.

README.md

@@ -2467,7 +2467,12 @@ looks for the document under `$XDG_DATA_HOME/help/<lang>/org.udpscope.app/` then
 `$XDG_DATA_DIRS/help/<lang>/org.udpscope.app/`, so with the set installed the alert names the missing
 *viewer* (`yelp`), and without it the alert names the identifier, the expected installed path and the
 install command — `cmake --install build --prefix ~/.local`, which writes
-`~/.local/share/help/C/org.udpscope.app/`. A build tree has neither the installed copy nor a
+`~/.local/share/help/C/org.udpscope.app/`. Two opt-in targets install the manual *alone*, without a
+whole `cmake --install`: `cmake --build build --target install-help-user` writes the same
+`$XDG_DATA_HOME` (default `~/.local/share`) copy the alert describes and needs no root, while
+`cmake --build build --target install-help` writes to the configured `CMAKE_INSTALL_PREFIX` for a
+packager or a system-wide user to wrap in `sudo`; both print where they wrote. A build tree has
+neither the installed copy nor a
 `help:` handler, and **either is enough to make `F1` do nothing**; that is the failure this names.
 `cmake --build build --target test-help` checks the set the way `test-ui` checks the `.ui` files:
 `yelp-check`'s offline subcommands (links, media, orphans, ids) and a `yelp-build html` render;