[Docs] Refresh and clarify remote thumbnail generation guide

[Copilot]

Summary

Expands the Remote Thumbnail Generation guide with the technical detail developers need when enabling the feature, based on a full read of the current implementation in shopware/shopware.

What changed

• Configuration table — added types and defaults for both remote_thumbnails.* keys, made enable: false default explicit.
• Pattern variables — clarified that {mediaUrl} resolves from shopware.filesystem.public and is replaced with an empty string for media whose path is already an absolute URL. Noted
  {mediaUpdatedAt} was added in 6.6.5.0.
• Private media warning — documented that private media is intentionally excluded from remote URL generation and keeps using its signed local URL.
• New "Behavior when enabled" section — documents what the flag actually short-circuits: ThumbnailService throws MEDIA_THUMBNAIL_GENERATION_DISABLED,
  GenerateThumbnailsHandler/MediaIndexer early-return, media:generate-thumbnails aborts, FileSaver skips dispatch and rename, MediaDeletionSubscriber skips thumbnail rows, MediaUrlLoader
  delegates to RemoteThumbnailLoader. Plus a warning that synthesized MediaThumbnailEntity IDs are random per request and not persisted.
• New "Cleaning up local thumbnails" section — documents bin/console media:delete-local-thumbnails (added 6.6.6.0) for migrating an existing shop.
• New "Extending the URL generation" section — documents the ResolveRemoteThumbnailUrlExtension hook (remote_thumbnail_url.resolve, since 6.7.1.0; supports returning null to skip a
  thumbnail since 6.7.3.0).
• Fastly section expanded — added soft_purge and max_parallel_invalidations to the example config; added a config table; clarified that api_key being non-empty is the on/off switch (no
  separate enabled flag); described the request shape and failure handling; added an info box disambiguating this media-cache node from shopware.http_cache.reverse_proxy.fastly, with a
  cross-link to the reverse-HTTP-cache guide.

[Micha Hobert (Isengo1989)]

Thx for the PR Martin Krzykawski (@MartinKrzykawski) 👍

[Copilot]

Pull request overview

This PR updates the Shopware developer documentation for Remote Thumbnail Generation to better reflect real runtime behavior, version availability, and operational considerations when switching from local thumbnail generation to remote/CDN-driven URLs.

Changes:

• Expanded configuration documentation with typed/defaulted config tables and clarified pattern variable behavior/versioning.
• Added new sections documenting what Shopware short-circuits when the feature is enabled, plus migration guidance for cleaning up existing local thumbnails.
• Significantly expanded the Fastly invalidation section (config options, activation behavior, and a cross-link clarifying the difference vs reverse-proxy Fastly config).

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