docs: update plugin development documentation

[0xtsukino]

Context

PR #276 adds a /create-plugin Claude Code command for scaffolding TLSNotary plugins. The command is published as a Claude Code marketplace plugin via .claude-plugin/ manifests. Several documentation areas need updating to reflect this and the current plugin development workflow.

Documentation to update

1. PLUGIN.md (root)

• Reference the /create-plugin command as the quickest way to get started
• Ensure the plugin API docs match current capabilities (doneWithOverlay, prove progress callbacks, etc.)
• Add link to example plugins in packages/demo/plugins/

2. Plugin SDK README (packages/plugin-sdk/README.md)

• Document doneWithOverlay() capability (added in feat(plugin-sdk): show success overlay before closing window #272)
• Document _proveProgress state for progress bar UI
• Document the Handler JSON path syntax with examples

3. Tutorial package (packages/tutorial/)

• Consider adding a step that uses the /create-plugin command
• Update any outdated API references (old createProver/sendRequest API vs unified prove())

4. Demo README (packages/demo/README.md)

• Document how to add new plugins to the demo (for contributors, not external users)
• Reference the shared @tlsn/plugins package and build pipeline

5. Claude Code marketplace listing

• Ensure .claude-plugin/plugin.json metadata is accurate
• Add screenshots or examples to the marketplace description if supported