[Doc] Replace the theme attribution in the nav footer

[koic]

The bottom of the sidebar (and the mobile page footer) showed the theme's fallback line "This site uses Just the Docs, a documentation theme for Jekyll", which tells readers nothing about the SDK. Overriding nav_footer_custom.html replaces it with the release the documentation describes, linking the RubyGems page of the version deployed via _data/versions.yml by bin/generate-gh-pages.sh, and with the licensing: the documentation is CC-BY-4.0 per LICENSE, while the code licensing is delegated to LICENSE itself, whose consent-based MIT to Apache-2.0 transition does not fit a footer phrase. Without versions data (local previews of the docs/ source) only the license sentences render.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed