Document `PluginMeta`'s `storage` feature in Apps developer reference

[Twixes]

Apps developer reference explains ~~plugin~~ app ecosystem features such as cache or attachments, but one feature notably is missing: storage. It needs to be added. The storage API can be found here in code: https://github.com/PostHog/posthog/blob/master/plugin-server/src/worker/vm/extensions/storage.ts