help: send readers to the install targets, in the alert and the manual

The failure alert told people to run `cmake --install build --prefix ~/.local`;
that still works, but `cmake --build build --target install-help-user` is the
form that needs no prefix guessing and prints the path it wrote, so the alert
names it and stays its former length. The manual gains a short "Installing
this manual" section on the overview page naming both targets -- the no-root
user one and the prefix-driven one for a system-wide or packaged install,
which the caller wraps in sudo -- and README and the GTK invariants doc follow
the alert's new command.

docs/claude/invariants-gtk-and-chrome.md

@@ -159,7 +159,8 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   `$XDG_DATA_HOME/help/<lang>/<id>/` then `$XDG_DATA_DIRS/help/<lang>/<id>/` for an `index.page` and
   the dialog says which one it is: with the manual there, install a viewer (for example `yelp`); with
   it absent, the identifier, the expected installed path and the install command
-  (`cmake --install build --prefix ~/.local`, which writes `~/.local/share/help/C/<id>/`) — and a
+  (`cmake --build build --target install-help-user`, which installs it under the user data
+  directory; `install-help` is the prefix-wide form for a system or packaged install) — and a
   viewer as well, the second possibility the error alone cannot rule out. Reporting a missing
   *document* as a missing *handler* is the defect that shape avoids.