docs: document Vitest SSR environment requirement for Astro 6

[Ntale3]

Description (required)

This PR documents a breaking change in Astro 6 where rendering .astro components in Vitest tests now requires using an SSR environment (node) instead of client environments (jsdom/happy-dom).

The temporary workaround added in Astro 5.x has been removed to ensure tests run in an environment that matches Astro's SSR build process.

What's included:

• Clear identification of who is affected (all three conditions must apply: Vitest + Container API + client environment)
• Explanation that users not rendering Astro components or already using node environment need no changes
• Step-by-step migration guide with code examples
• Workspace configuration example for projects needing multiple test environments

Related issues & labels (optional)

• Closes Don’t allow rendering Astro components in Vitest “client” environments astro#14895
• Suggested label: improve documentation

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[Ntale3]

@delucis
I'd really appreciate your guidance and review on this PR. If there's anything that needs adjustment to better match the docs style or if I've missed any important details, please let me know. Happy to iterate on this!