feat(i18n): document new option

[ematipico]

Description (required)

Closes PLT-1431

This PR adds documentation for the upcoming new option. I added a small chapter, I don't know if I need to repeat the examples. Let me know what you think.

Related issues & labels (optional)

• Closes #
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For Astro version: 4.2. See astro PR #9638.

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[VoxelMC]

Hello! Thank you for making sure this is documented!

I have added some potential alternative phrasing, should you desire to choose one.
I hope this helps!

[sarah11918]

Tried adding a tiny bit to the section directly above to help with the issue where people are forgetting to make an index page when prefixDefaultLocale is set to true:

• highlights, and adds an inline comment for index.astro
• moves the second bullet point down and expands on what this option means (including the file NAME, not just the root URL)

[sarah11918]

@ematipico - See what you think about these changes. Since I knew you were also adding an error message about a missing index.astro file, I figured we could help the existing documentation here too, at the same time.

Please check the additions below and edit for correctness as necessary. e.g. I invented a reason why someone might configure this as false, to give more context. I think it's ok to expand this, but I'd like to make sure this is accurate/gives the right nuance!

[ematipico]

Thank you, Sarah; I like very much the new phrasing!

[sarah11918]

Note to self: ADD A LINK TO THE CONFIG REFERENCE ITEM

But otherwise, happy with the wording!

[sarah11918]

Link will fail until #6437 is merged.