[doc] Enable gtkdoc-check

Should catch the most blatant issues.
diff --git a/docs/meson.build b/docs/meson.build
index e0b0e36..10d9250 100644
--- a/docs/meson.build
+++ b/docs/meson.build
@@ -59,4 +59,6 @@
   html_assets: html_images,
   ignore_headers: ignore_headers,
   dependencies: [libharfbuzz_dep],
-  install: true)
+  install: true,
+  check: true,
+)