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GH-41405: [Release][Docs][GLib] Use Sphinx based GLib front page (#41406)

### Rationale for this change

We should use the GLib front page generated by Sphinx.

### What changes are included in this PR?

Stop reverting the GLib front page change in release script.

### Are these changes tested?

No.

### Are there any user-facing changes?

Yes.
* GitHub Issue: #41405

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