gtk-doc: Ensure we have recent enough version

Older versions won't produce errors, but will silently miss documentation for a few symbols.

With Meson 0.52.0 (not yet released) and gtk-doc 1.33 (not yet released) we can fetch gtk-doc automatically if it's not installed on the system or version is too old. This has the side effect of making much easier to build the doc on other platforms than Linux, where it's more difficult to install dependencies.