Existing Astro Topic Update

learn/astro/ is still valuable as the general Astro guide, but it now needs a version-framing update.

Recommended Follow-Up

Update learn/astro/README.md after this topic lands:

• Change “Current version: Astro 6.1” to Astro 7.
• Replace astro-6 tag with astro-7 or add both only if the topic still intentionally covers Astro 6 specifics.
• Add a link to ../astro-7/README.md for migration and Astro 7-specific changes.
• Re-check notes/gotchas.md and concepts/rendering.md for route caching language that still says experimental.
• Re-check concepts/content-collections.md and Markdown-related pages for Sätteri/unified assumptions.

What Not To Do

Do not merge all Astro 7 content into learn/astro/. Keep the general guide stable and send version-specific upgrade details here.