enhance(toc): localize "In this article" header

[caugner]

[!IMPORTANT]
Currently, we do not accept any community PRs for UI localization. We will revisit UI localization later this year or early next year. Thank you for your understanding!

Summary

(MP-1194)

Problem

Our user interface isn't localized, but some content UI parts should be localized like macros.

Solution

Localize the "In this article" header that is shown above the TOC.

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Screenshots

Before

After

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How did you test this change?

Ran yarn dev and checked http://localhost:3000/fr/docs/Web/HTML locally.