chore(.markdownlint.yaml): use ordered style for MD029

[kenji-miyake]

Description

To use consistent styles, I think it's good to use ordered for MD029.
The default value is one_or_ordered, and it allows two different styles.

For more details, please refer to https://github.com/DavidAnson/markdownlint/blob/main/doc/Rules.md#md029---ordered-list-item-prefix.

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[kenji-miyake]

With this change, the following style isn't allowed.

We need to fix it to the following style. If you write 2., it will be automatically formatted.

[xmfcx]

I agree with the ordered style as long as it is automatically enforced, fixed.