docs: Instructions for partners & contributors of external plugins

## Context

Following the addition of external plugin support in #50, we need documentation for partners and contributors who want to create external plugins for git-ape.

## Requirements

- Document the plugin metadata format and required fields
- Explain how to register a plugin in the `/plugins` configuration
- Provide a step-by-step guide for submitting a new external plugin
- Include examples based on the `ape-context` plugin as a reference

## Related

- PR #50 - feat(plugin): add ape-context plugin for enhanced context management