docs: refresh multilingual content authoring guides

[Dawn6666666]

Type of change

• Bug fix (a non-breaking change that fixes an issue)
• New feature (a non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other: documentation and example content maintenance

Checklist

• I have read the contribution guidance referenced by the repository template.
• I have checked to ensure that this Pull Request is not for personal-only changes.
• I have performed a self-review of my own changes.
• I have run the repository checks; intentionally not run for this documentation-only request.

Related Issue

Closes #545

Changes

• Refresh the English, Simplified Chinese, Japanese, and Traditional Chinese README content-authoring entry points.
• Add multilingual content-authoring guides covering current Frontmatter, Markdown/MDX extensions, media, diagrams, encryption limits, synchronization caveats, and publishing reminders.
• Link the guides to real examples under src/content/posts/, including standard Markdown, extensions, Mermaid, image grids, video, encryption, folder-based posts, and MDX.
• Update example posts with a current generic folder/frontmatter example, a valid video embed example, an explicit browser-side encryption demo with the repository's public demo credential, a generic MDX link example, and a data-no-enhance demonstration.
• Keep the extension showcase article's Mizuki-specific values so it remains a project feature demonstration.
• No runtime theme code was changed.

How To Test

Per request, no checks, tests, or builds were run. I performed a read-only review of the documentation and content diffs.

Screenshots (if applicable)

Not applicable; this PR changes documentation and demo content only.

Additional Notes

The repository's existing demo posts, page data, albums, and images are still demo content and should be removed or replaced before deploying a personal site.

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