Add info about using @latest with remote-theme

[bedroesb]

https://github.com/benbalter/jekyll-remote-theme?tab=readme-ov-file#declaring-your-theme

To automatically use the latest tagged release, you can specify @latest (e.g., benbalter/retlab@latest). This will fetch the most recent release from the GitHub Releases API. If no releases exist, it will fall back to using HEAD.