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 storages every project starts with, choosing access, using storage from server code, managing storages and their files from the CLI and the console, and what happens at a plan limit.

The blockers listed on this PR are resolved. The provider API shipped in Serverpod 4.0.0, projects are created with both default storages, and the console Storage tab is live and no longer gated. The page is updated against Serverpod 4.0.0 and Cloud CLI 1.0.0:

• Server-side wiring uses ServerpodCloudProvider.private() and .public(), which serverpod create already adds to new projects, and the session.storage examples use the shipped method names.
• Adds the serverpod cloud storage commands, which the first draft did not cover, including creating and deleting a storage.
• Keeps custom storages reachable from server code, and says plainly that Cloud connects the two defaults for you while a storage you add has no ready-made helper, with a pointer to how storage providers are registered.
• Limits are scoped to the plans that set caps, and describe what a lockout affects and how access comes back.
• Treats private and public as storage ids named after their access rather than as fixed categories.

The console workflow, its labels, the storage id rules, and the type-the-storage-id delete confirmation were all checked against source.

[developerjamiu]

Reworked against Serverpod 4.0.0 and Cloud CLI 1.0.0.

• The placeholder class is now ServerpodCloudProvider.private() and .public(), and three session.storage method names are corrected.
• Added the serverpod cloud storage commands.
• Limits now scope the lockout to the plans that set caps.

Your blockers are resolved: the provider API shipped, projects get both default storages, and the Storage tab is live and ungated.

I first cut the custom-storage registration step, thinking server code could reach only private and public. That was wrong, so it is back, worded to say there is no ready-made helper for a custom storage id.

[SandPod]

Have a couple of questions for you before I approve.

I assume that all has been tested with the latest version of the CLI/Console.

[Isakdl]

Looking good, but added some clarification comments on some section that I think needs a rework.

[SandPod]

Looks good from my perspective.

But still unsure about the code examples from the Framework. Easy that this gets outdated. Perhaps this is better suited for a guide?

[developerjamiu]

The code left here is Cloud's own wiring, not the framework API. The API lives on the file-uploads page (#857), and the CDN and passwords pages carry the same kind of snippet.