feat: show v6 migration guide in sidebar

[trueberryless]

Description (required)

Add v6 migration guide to the sidebar, where the other major guides are 👍
(Also fixing a lonely, sad and forgotten opening bracket 🥹)

Related issues & labels (optional)

• Astro v6.0 #12322
• Suggested label: v6 or something 🙌

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Warnings reference

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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.

[sarah11918]

Thank you! Great catch (on both).