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GH-41256: [Format][Docs] Add a canonical extension type specification for JSON (#41257)

### Rationale for this change

As per #41256 this proposes a specification of a canonical extension type for JSON serialized data.

### What changes are included in this PR?

This adds to documentation of canonical extension types.

### Are these changes tested?

No as only docs are changed.

### Are there any user-facing changes?

No.
* GitHub Issue: #41256

Lead-authored-by: Rok Mihevc <rok@mihevc.org>
Co-authored-by: Will Jones <willjones127@gmail.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: Rok Mihevc <rok@mihevc.org>