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GH-39935: [GLib][Docs] Use GI-DocGen instead of GTK-Doc (#40427)

### Rationale for this change

DI-DocGen is a new documentation generator for GObject-based libraries. GTK-Doc is still maintained but abandoned.

### What changes are included in this PR?

Use GI-DocGen.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* GitHub Issue: #39935

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>