Add SDK migration tutorial for sfcc-ci programmatic API users

New guide at docs/guide/sdk-migration.md covering the migration from
sfcc-ci's require('sfcc-ci') JavaScript API to @salesforce/b2c-tooling-sdk.
Includes side-by-side code examples, paradigm shift explanations
(callbacks→async/await, token passing→config-based auth, untyped→typed),
per-module API mapping, and a comprehensive reference table.

Also adds cross-references from the CLI migration guide and SDK README.

Author: clavery

.changeset/sdk-migration-tutorial.md

@@ -0,0 +1,5 @@
+---
+'@salesforce/b2c-dx-docs': patch
+---
+
+Add SDK migration tutorial for sfcc-ci programmatic API users

docs/.vitepress/config.mts

@@ -44,6 +44,7 @@ const guidesSidebar = [
       {text: 'Authentication Setup', link: '/guide/authentication'},
       {text: 'CI/CD with GitHub Actions', link: '/guide/ci-cd'},
       {text: 'sfcc-ci Migration', link: '/guide/sfcc-ci-migration'},
+      {text: 'sfcc-ci SDK Migration', link: '/guide/sdk-migration'},
       {text: 'Account Manager', link: '/guide/account-manager'},
       {text: 'Analytics Reports (CIP/CCAC)', link: '/guide/analytics-reports-cip-ccac'},
       {text: 'IDE Integration', link: '/guide/ide-integration'},