📚 docs(document): add getting-started pages for `core` and `hooks` packages

[acasazza]

Summary

Add two new MDX documents to packages/document/src/stories/getting-started/ describing the core and hooks internal packages — their purpose, API, and usage patterns.

Motivation

The getting-started section currently covers introduction, authentication, microfrontends, styling, and containers (001 – 005). Contributors and consumers who want to understand or extend the library have no reference for:

• @commercelayer/core — low-level SDK wrappers (fetch functions, types)
• @commercelayer/hooks — SWR-based React hooks built on top of core

Tasks

• Create 006.core.mdx — document the @commercelayer/core package

  • What it is and when to use it directly
  • Available functions (getSkus, retrieveSku, getPrices, getSkuLists, etc.)
  • Function signature pattern ({ accessToken, params })
  • TypeScript types exported
  • Code example

• Create 007.hooks.mdx — document the @commercelayer/hooks package

  • What it is and when to use it directly
  • Available hooks (useSkus, usePrices, useSkuLists, etc.)
  • SWR-based caching and revalidation behaviour
  • Return shape (data, isLoading, error, fetch/clear actions)
  • Code example

Notes

• Follow the existing MDX structure in the getting-started/ folder (title, description, prop/API tables, code blocks with syntax highlighting)
• Both pages should be linked from the Storybook sidebar under "Getting Started"