docs: add agentic files guide

[Jonas Elfering (keulinho)]

Summary

• Add a Storefront template guide for the new agentic files feature.
• Document default files, public path derivation, template extension patterns, custom file templates, and Administration description snippets.
• Explain Agentic Commerce plugin compatibility, including core-feature mode, fallback serving on older Shopware versions, and automatic enablement for UCP-enabled sales channels.

Related links

• Related platform feature: feat: add sales channel files shopware#17143

Checklist

• I reviewed affected links, code samples, and cross-references, including PageRef references where relevant.
• I added or updated redirects in .gitbook.yaml if pages were moved, renamed, or deleted.
• I updated .wordlist.txt (and sorted it) if spellcheck flags new legitimate terms.
• Any required dependent changes in downstream modules have already been merged and published.
• This pull request is ready for review.

Notes

No redirects are required because this PR only adds a new page. Local validation: git diff --check.