[i18nIgnore] fix: i18n-guides/Japanese.md

[Yusaku01]

Description (required)

• Colons are used fairly frequently in the English version, but the Japanese translation guide
  did not have an explicit rule on whether to keep them as colons or replace them with something else.
• In the existing Astro Docs Japanese translations, colons tend to be replaced with periods
  (。), though a fair number of colons still remain.
• While this may seem like a minor difference to readers, having to mentally equate colons and
  periods while reading adds unnecessary cognitive load.
• There may be cases where a colon is appropriate, but in Japanese, the following approaches
  feel more natural:
  • Replacing with a period (。)
  • Supplementing with words like "以下..." (below/following) to direct the reader's attention
    to what comes next

For the reasons above, I created this PR.
I realize the timing may not be ideal given the ongoing v6 preparation, but without an explicit rule, we risk continuing to produce inconsistent translations that create a stressful reading experience for users🤔

Additional info

It might be worth illustrating this rule with conforming and non-conforming examples, similar to the "Spaces around alphanumeric characters (### 英数字前後のスペース)" section.

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[Yusaku01]

I've moved this to draft for now, as I don't think it's urgent.

[morinokami]

ありがとうございます！一点だけコメントしましたのでご確認をお願いします 🙏

[morinokami]

この記述自体は問題ないと思いますが、下部にインデントして例を補ってあげたほうがよりわかりやすいかなと思いました（「たとえば〜という英文は、〜ではなく〜のように訳します」みたいな感じで）。