docs: add migration guide from official SDK

[looooown2006]

Summary

Adds a migration guide for teams moving from the older official PHP SDK, sveaekonomi/checkout, to nordkit/svea.

The guide covers:

• why migrate;
• configuration mapping from the old connector setup to config/svea.php / environment variables;
• side-by-side examples for checkout, admin, refund/cancel, and webhook flows;
• response mapping and migration gotchas;
• testing the migrated flow with Svea::fake().

It also wires the page into the VitePress sidebar under Guide -> Migration.

Verification

• git diff --check
• npm run build in docs/
• npx --yes markdown-link-check docs/guide/fluent-builders.md
• gh repo view sveawebpay/php-checkout --json nameWithOwner,url,defaultBranchRef

Note: local markdown-link-check reached all internal migration-guide links but returned Status: 0 for the external GitHub URL in this network environment. The URL was separately verified with gh repo view.

Closes #3.