Finish `jekyll-github-metadata` emulation

[osteele]

The jekyll-github-metadata plugin populates site.github with metadata about the repository when building GitHub Pages sites. This includes project name, description, URLs, owner information, and more.

Gojekyll has partial implementation in plugins/github_metadata.go, but several features are still missing:

Missing features:

• contributors - List of repository contributors
• public_repositories - List of owner's public repositories
• show_downloads - Whether downloads are shown
• releases - Repository releases
• versions - Version information
• wiki_url - URL to the repository wiki

Also not implemented:

• Octokit configuration options
• GitHub Enterprise support

Current implementation:
The plugin currently provides basic repository metadata like name, description, clone URLs, owner info, and repository URLs. See plugins/github_metadata.go lines 66-68 for the list of missing features.

Reference: Listed as "partial" implementation in docs/plugins.md line 14