docs(cloud): Add a storage concept page

[Isakdl]

Adds cloud_docs/concepts/storage.md, a concept page for Serverpod Cloud storage.

It covers the two default storages a new project gets, choosing private vs public, registering a storage on the server, adding one in the console, browsing and uploading files, deletion, and plan limits.

Blockers before this can leave draft

• ServerpodCloudStorage does not exist yet. The two registration snippets use an invented API, pod.addCloudStorage(await ServerpodCloudStorage.create(storageId: 'private')), and carry TODO markers in HTML comments. The shape deliberately mirrors the NativeGoogleCloudStorage.create block already in the framework file-uploads page, and takes only a storage id because Cloud already knows the bucket name, region, and visibility. No package or import is named, because there is no basis to guess either. Replace both blocks with the real class before merging.
• The default private and public storages have not shipped. The page states as present tense that every new project starts with both, matching the framework template.
• The console tab is hidden in production. showBuckets: !ref.watch(configProvider).isProduction. This should merge with the release that flips it.
• The page uses the renamed console labels (Storage tab, Create storage, Delete storage). The console source still says Buckets, Create bucket, and Delete bucket. These need to match before merge, in one direction or the other.

Notes

• The countable noun is "a storage", not "a bucket", matching the framework's own vocabulary (storageId, session.storage, "Serverpod comes with two default storages").
• Exact plan figures are deliberately absent. The page links to Serverpod Cloud plans instead, the same way database-backups.md does, because the numbers live in project_capsule_products.dart and drift.
• The page says to run scloud deploy after creating a storage. The console Redeploy button re-sends the currently active managed-secret version rather than advancing to the latest, so it does not pick up a new storage. The page avoids naming that button for this reason.

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https://claude.ai/code/session_01N237K5XruApUeGphztg8nc