meson: Require gi-docgen when gtk_doc is required

meson: Require gi-docgen when gtk_doc is required

Fixes build with only `-Dgtk_doc=true` without
`-Dintrospection=enabled`:

    Program gi-docgen found: NO
    Configuring gdk4.toml using configuration

    docs/reference/gdk/meson.build:13:2: ERROR: Tried to use not-found external program in "command"

`introspection` is `auto` by default.

The rest of the docs build is only contingent on
`if get_option('gtk_doc')` so we should use the same restriction here.

We check for `build_gir` below already, and the gi-docgen subproject
itself does not need gobject-introspection so it's fine to do it like
this.