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.

cmake/install_help_user.cmake

@@ -0,0 +1,44 @@
+# SPDX-License-Identifier: MIT
+#
+# The `install-help-user` target's body. Copies the Mallard help set into the user's XDG data
+# directory -- where `help:<id>` resolves without root -- and prints every path it wrote.
+#
+# `$XDG_DATA_HOME` is honoured when it is set, and `$HOME/.local/share` is the fallback when it
+# is not: that is the XDG Base Directory Specification's own stated default for the variable, and
+# it is the default `help_installed_path()` falls back to as well, so the manual lands where the
+# application and the desktop viewer look for it. The variable names a *data directory*, not a
+# prefix -- its `share/` is not appended -- so the set goes to `<data_home>/help/C/<id>/`. An
+# arbitrary `$XDG_DATA_HOME` need not end in `share`, which is exactly why this copies the tree
+# rather than deferring to `cmake --install --prefix`, whose datadir is always `<prefix>/share`.
+#
+# Like the desktop-entry target, this runs only when it is named: `cmake --build build` never
+# touches the user's data directory.
+#
+# Expects -DHELP_SRC= and -DHELP_NAME= on the command line.
+
+if(DEFINED ENV{XDG_DATA_HOME} AND NOT "$ENV{XDG_DATA_HOME}" STREQUAL "")
+  set(data_home "$ENV{XDG_DATA_HOME}")
+elseif(DEFINED ENV{HOME} AND NOT "$ENV{HOME}" STREQUAL "")
+  set(data_home "$ENV{HOME}/.local/share")
+else()
+  message(FATAL_ERROR
+    "XDG_DATA_HOME and HOME are both unset or empty; cannot determine the user data directory")
+endif()
+
+if(NOT DEFINED HELP_SRC OR NOT DEFINED HELP_NAME)
+  message(FATAL_ERROR "HELP_SRC and HELP_NAME are both required")
+endif()
+
+set(dest "${data_home}/help/C/${HELP_NAME}")
+
+# The same filter as the install() rule, so both destinations carry the same tree: the pages and
+# the figures, and nothing an editor left behind.
+file(COPY "${HELP_SRC}/" DESTINATION "${dest}"
+     FILES_MATCHING PATTERN "*.page" PATTERN "*.png" PATTERN "*.jpg")
+
+# Printed because there is no uninstall target: undoing this is these paths.
+file(GLOB_RECURSE installed LIST_DIRECTORIES FALSE RELATIVE "${dest}" "${dest}/*")
+list(SORT installed)
+foreach(path IN LISTS installed)
+  message(STATUS "wrote ${dest}/${path}")
+endforeach()