fix(tutorial): prevent character encoding issue in unit 4

[ArmandPhilippot]

Description (required)

Since the change introduced by withastro/astro#12231 in Astro 5, <meta charset="utf-8" /> is no longer automatically added for Markdown files that use the layout frontmatter property. So users was facing a character encoding issue in unit 4 while checking the rendering of MarkdownPostLayout before nesting it in BaseLayout.

Before	After

This PR:

• adds <meta charset="utf-8" /> in unit 4-2
• adds a new step in unit 4-3 to remove the <meta charset="utf-8" /> (I thought maybe it was easier for beginners to understand rather than removing it in step 1)

Related issues & labels (optional)

• Closes /tutorial/4-layouts/2: chinese are garbled characters in the posts/post-* while using MarkdownPostLayout #10977
• Suggested label: improve or update documentation, tutorial

[astrobot-houston]

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🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

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[sarah11918]

I hate that we need this 😅

[ArmandPhilippot]

Yeah, I know... at least it's one line, we don't need the whole page shell. 😅