docs: End-user documentation for marketplace & plugin installation

[suuus]

Context

Following the addition of external plugin support in #50, we need end-user facing documentation explaining the marketplace and how to install plugins.

Requirements

• Explain what the git-ape marketplace is and how it works
• Document how users can browse available plugins
• Provide installation instructions for external plugins
• Cover plugin configuration and usage basics

Related

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