Add documentation example: integrating an external CMS with CF

[mdanilowicz]

Create a clear, practical documentation page (with working code samples) that demonstrates how to integrate an external headless CMS with the shopware/frontends framework. The example should show a minimal, production-minded integration that maps external CMS content to the frontend's CMS-component model, supports SSR, media handling, and caching considerations. The goal is to provide a reference pattern implementers can copy or adapt for Contentful/Strapi/Prismic/Netlify CMS-like systems.

Current Behavior

There is no official example or documentation page in the repository showing how to integrate an external CMS into the shopware/frontends architecture. Contributors must piece together integration patterns from disparate examples or internal knowledge.

Acceptance Criteria

• New documentation page added at docs/guides/integrate-external-cms.md (or examples/external-cms/README.md) containing the sections listed in Possible Solution.

• At least one minimal, runnable example included under examples/external-cms/ that can be started with pnpm install && pnpm dev and demonstrates fetching + rendering content (mock CMS or free-tier CMS configuration documented).