Release notes were written into the GitHub release body, which means they exist
once, in English only, and are awkward to revise after publishing. Putting them
on the site gives each release a Japanese and an English page that can be
corrected later and linked from anywhere.

The overview page states the versioning rules the version numbers alone do not
convey: that dropping a supported platform is a MINOR bump, that the Paper and
Velocity builds advance separately, and that the plugin version says nothing
about compatibility.

Co-Authored-By: Claude <noreply@anthropic.com>

- Add download page with GitHub API data loader for latest releases
- Add DownloadCard Vue component with JA/EN i18n support
- Include Modrinth and CI build links
- Rename docs/ directory to website/

Co-Authored-By: Claude <noreply@anthropic.com>