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GH-35084: [Docs][Format] Add how to change format specification (#35174)

### Rationale for this change

It's for easy to refer.

### What changes are included in this PR?

This is based on the discussion:

[DISCUSS] Format changes: process and requirements
https://lists.apache.org/thread/9t0pglrvxjhrt4r4xcsc1zmgmbtr8pxj

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* Closes: #35084

Lead-authored-by: Sutou Kouhei <kou@clear-code.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Co-authored-by: Will Jones <willjones127@gmail.com>
Co-authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>