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GH-40616: [Docs][GLib] Ensure overwriting placeholder front pages (#40618)

### Rationale for this change

Place holder font pages are still used because Sphinx build is executed before GLib docs build.

### What changes are included in this PR?

Build Sphinx docs before GLib docs are built.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* GitHub Issue: #40616

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