fix(i18n): add callout clarifying write-translations only handles JSO…

[MasterIfeanyi]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Users running the write-translations command often expect it to copy their Markdown files to the i18n folder, but it only generates JSON translation files for UI strings and plugin data. This causes confusion, especially for new users setting up multi-language sites.

This PR addresses #8703 by:

1. Adding a console message after write-translations runs, clearly stating the
   command only handles JSON files and linking to Markdown translation docs
2. Adding a :::note callout in the i18n tutorial at the exact point of confusion,
   signposting users to the "Translate Markdown files" section

Closes #8703

Test Plan

Code change:

• Ran pnpm start to build the project
• The console message appears after the write-translations command completes
• Message includes a working link to the i18n tutorial

Docs change:

• Verified the new :::note callout renders correctly on the local dev server
• The internal link #translate-markdown-files correctly scrolls to the right section
• Tested at http://localhost:3000/docs/i18n/tutorial

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

Closes #8703

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