doc: Fix the installation of manuals

The manuals shouldn't be installed in the top-level directory, but in
one of the sub-directories corresponding to the relevant section.

Fallout from 0a972dfccc

doc/meson.build

@@ -22,7 +22,7 @@ foreach manual: manuals
     command: go_md2man_command,
     input: input,
     install: true,
-    install_dir: get_option('mandir'),
+    install_dir: join_paths(get_option('mandir'), 'man1'),
     output: output,
   )
 endforeach