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

[acasazza]

Summary

Closes #737

Add two new MDX pages to the Storybook getting-started section describing the internal core and hooks packages.

New pages

006.core.mdx — @commercelayer/core

• What it is and when to use it directly (no React dependency)
• Full table of exported functions (getSkus, retrieveSku, updateSku, getPrices, retrievePrice, updatePrice, getSkuAvailability, getSkuLists, retrieveSkuList, getAccessToken)
• Consistent function signature pattern ({ accessToken, params, options })
• Code examples: authentication, filtered SKU list, SKU list with included SKUs

007.hooks.mdx — @commercelayer/hooks

• What it is and when to use it directly
• Full table of exported hooks (useSkus, usePrices, useSkuLists, useAvailability)
• Return shape reference table (data, isLoading, isValidating, error, fetch/retrieve/update/clear actions, mutate)
• SWR caching behaviour explained
• Code examples: useSkus, useSkuLists, usePrices, useAvailability

Test plan

• pnpm storybook starts without errors in packages/document
• Storybook sidebar shows "Getting Started / Core package" and "Getting Started / Hooks package"
• All tables, code blocks, and callout spans render correctly