docs: clarify yearly-only rule for dify education discount

[RiskeyL]

Summary

• Users were selecting the Monthly Professional plan and getting charged $59 because the yearly-only requirement for the education discount was buried in Step 3. This PR rewrites the "Dify for Education" section so the Yearly-plan requirement is unmissable and adds "Why was I charged $59?" as the lead FAQ.
• Fully syncs zh and ja with en: both previously lacked the Education section entirely, carried stale sections no longer in en, and had mistranslations (most notably ワークフロー used for "workspace" in 5 spots).
• Codifies the $ escape rule in formatting-guide.md and the billing UI labels in glossary.md so this page renders correctly and future writers follow the same canonical strings.

[Copilot]

Pull request overview

Updates the Billing/Subscription Management documentation to make the “Education discount is yearly-only” requirement unmissable, prevent users from accidentally selecting Monthly Professional, and align terminology and translations across locales.

Changes:

• Rewrites the “Dify for Education” section (and FAQ ordering) to foreground the yearly-only requirement and the “Why was I charged $59?” explanation.
• Renames the Free plan references to “Sandbox” and updates plan comparison tables accordingly across EN/JA/ZH.
• Adds canonical UI labels (Billing-related) to glossary.md and documents the $ escaping rule in formatting-guide.md.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
en/use-dify/workspace/subscription-management.mdx	Clarifies yearly-only education discount flow, updates plan naming to Sandbox, improves UI-path formatting.
ja/use-dify/workspace/subscription-management.mdx	Syncs JA with EN structure/content, fixes workspace terminology, adds Education section + FAQ.
zh/use-dify/workspace/subscription-management.mdx	Syncs ZH with EN structure/content, updates plan naming/table, adds Education section + FAQ.
writing-guides/glossary.md	Adds canonical Billing UI labels/keys (plans + Upgrade) and “Get Education Verified”.
writing-guides/formatting-guide.md	Codifies $ escaping guidance for Mintlify/remark-math and adds checklist item.

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