docs(v0.2): migration guide and API reference

[genu]

Part of #179.

Document the v0.2 convention-based architecture and provide a migration path for users on the old plugin-object API.

Scope

• New docs page: "Server integration" — covers convention files, `defineUploadServerConfig`, `defineUploadConfig`, auto-mounted endpoints, `useServerUpload`.
• New docs page: "Migration from v0.1" — shows side-by-side old `PluginS3` + hand-rolled presign route → new server config.
• Update provider pages for S3, Azure, Firebase to show both paths (new server adapter + legacy escape hatch).
• Update the main README and getting-started guide.

Acceptance

• A new user can follow getting-started and upload to S3 without reading external SDK docs.
• An existing v0.1 user can follow the migration guide and port their app.