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.

help/C/org.udpscope.app/overview.page

@@ -167,4 +167,20 @@
     </list>
   </section>
 
+  <section id="installing">
+    <title>Installing this manual</title>
+    <p>The manual is a help set the desktop viewer reads off disk, so <key>F1</key> opens it only
+    once a copy sits where <cmd>help:</cmd> resolves. Either of two targets installs it on its own,
+    without building the application, and each prints where it wrote:</p>
+    <list>
+      <item><p>For your own account, needing no root:
+      <cmd>cmake --build build --target install-help-user</cmd> writes it under
+      <file>$XDG_DATA_HOME</file> (default <file>~/.local/share</file>).</p></item>
+      <item><p>For a system-wide or packaged install:
+      <cmd>cmake --build build --target install-help</cmd> writes it under the configured
+      <cmd>CMAKE_INSTALL_PREFIX</cmd>, so point that at <file>/usr</file> and wrap the target in
+      <cmd>sudo</cmd>.</p></item>
+    </list>
+  </section>
+
 </page>