feat(i18n): support translating through locale extensions `myDoc.fr.md`

[slorber]

Motivation

Translating through the i18n folder is nice for i18n SaaS integrations (Crowdin)

But it is not ideal for simpler needs based on Git, as it requires to explain the i18n folder structure convention.

To make i18n simpler to adopt, let's add support for localized file extensions to all our core content plugins

• ./docs/myDoc.fr.md
• ./blog/myBlog.fr.md
• ./src/pages/myPage.fr.tsx

This kind of convention is already used by other documentation frameworks:

• https://ultrabug.github.io/mkdocs-static-i18n/getting-started/quick-start/
• https://nextra.site/docs/guide/i18n#add-locale-to-filenames

Note, we also want to support this for metadata files:

• docs _category_.fr.yml
• blog authors.fr.yml

Note, the editUrl and editLocalizedFiles options should rather work as expected too

Test Plan

Unit tests + dogfood

Test links

https://deploy-preview-9700--docusaurus-2.netlify.app/

Local:

• http://localhost:3000/fr/tests/pages/i18n
• http://localhost:3000/fr/tests/blog/i18n
• http://localhost:3000/fr/tests/docs/i18n

[netlify[bot]]

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[github-actions[bot]]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 65	🟢 98	🟢 100	🟢 100	🟠 89	Report
/docs/installation	🟠 69	🟢 98	🟢 100	🟢 100	🟠 89	Report
/docs/category/getting-started	🟠 75	🟢 100	🟢 100	🟢 90	🟠 89	Report
/blog	🟠 75	🟢 100	🟢 100	🟢 90	🟠 89	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 65	🟢 97	🟢 100	🟢 100	🟠 89	Report
/blog/tags/release	🟠 74	🟢 100	🟢 100	🟠 80	🟠 89	Report
/blog/tags	🟠 75	🟢 100	🟢 100	🟢 90	🟠 89	Report

[github-actions[bot]]

Size Change: +1.84 kB (0%)

Total Size: 929 kB

Filename	Size	Change
website/.docusaurus/globalData.json	59.5 kB	+317 B (+1%)
website/build/assets/js/main.********.js	718 kB	+1.52 kB (0%)

ℹ️ View Unchanged

Filename	Size
website/build/assets/css/styles.********.css	114 kB
website/build/index.html	37.9 kB

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