feat(website): Parse plugins JSON and list extensions on website

## Context

Following the addition of external plugin support in #50, the git-ape website should dynamically list available extensions by parsing the plugins JSON configuration file.

## Requirements

- Parse the external plugins JSON file from `/plugins`
- Display available plugins on the website with metadata (name, description, author, etc.)
- Keep the listing in sync with the repository source of truth
- Include links to plugin documentation and installation instructions

## Related

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