docs: Instructions for partners & contributors of external plugins

[suuus]

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