arrow

GH-40441: [GLib][Docs] Use Sphinx for Apache Arrow GLib front page (#40442)

### Rationale for this change

Using both of apache/arrow and apache/arrow-site to maintain https://arrow.apache.org/docs/c_glib/  is difficult to maintain.

### What changes are included in this PR?

Use  https://github.com/apache/arrow/tree/main/docs/source/c_glib as https://arrow.apache.org/docs/c_glib/ 's source.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* GitHub Issue: #40441

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