DOC: Add store import changes to v3 migration guide (fix #2733)

[OPMTerra]

My first contribution addressing #2733! This PR adds documentation to clarify store import changes in the v3 migration guide.

What's Included

• Clear comparison table showing v2 vs v3 imports for MemoryStore/DirectoryStore
• Example using tempfile.TemporaryDirectory for TempStore migration
• Fixed formatting in the migration guide based on build feedback

Checklist (For Maintainers)

✅ Docs updated in user-guide/v3_migration.rst
✅ No code changes needed (pure documentation)
✅ Built docs locally to verify formatting

This helps users like those in imagecodecs#123 who encountered build errors with old imports. While it doesn't fix external projects directly, it:

1. Shows proper v3 import paths
2. Explains replacements for deprecated stores
3. Provides copy-paste examples for common migrations

I'm happy to make any adjustments needed!
@OPMTerra

[dstansby]

This is a great addition! My one suggestion before merging is to change the heading level so the new heading appears under "The Store class" level.

[OPMTerra]

Hi @dstansby 👋,

I've adjusted the heading level as suggested. Let me know if anything else needs tweaking!

Thank you for the review! 🙏