docs: Install the gi-docgen docs

They are now installed to (e.g.)
`${prefix}/share/doc/glib-2.0/{glib,gmodule,gobject,gio}/index.html`.

We might want to drop one level of nesting out of that, but for the
moment I thought I’d keep it in so we can disambiguate by installed
major version.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3037

docs/reference/gio/meson.build

@@ -54,4 +54,7 @@ custom_target('gio-docs',
   ],
   build_by_default: true,
   depend_files: expand_content_files,
+  install: true,
+  install_dir: docs_dir,
+  install_tag: 'doc',
 )