This ensures that the Doxygen-built documentation has the same layout
in the installed files that it does in the build tree and on the
website. If we don't keep the same layout, then there is no value for
the `DBUS_APIDOC_LINK` in index.html that would be correct for both
the build tree and the installed tree. The build tree effectively has
a html subdirectory hard-coded, because that's how Doxygen lays out
its outputs.
This commit is the Meson equivalent of
commit 522633b4 "cmake: install api docs in html subdir" in the CMake
build system (dbus!473, dbus#519).
Signed-off-by: Simon McVittie <smcv@collabora.com>
