Auto-detect platform for marketing website download button

The [website download button](https://github.com/pingdotgg/t3code/blob/v0.0.3/apps/marketing/src/pages/index.astro#L41-L48) shouldn't hard-code a link to the GitHub releases page. Instead, it should auto-detect the user's platform and download the appropriate package for them from the latest (stable?) release.