feat: add since shortcode for release badges

Migrate the `since` shortcode from the documentation site into the theme
so every Ananke site can use it. It renders a small badge linking to the
release a feature was introduced in.

The version is resolved from the first positional argument, a named
`version` argument, or the page's `since` front matter. The release link
is built from a configurable printf format string
(`params.ananke.shortcodes.since.url`) and the label uses a new
translatable `since` i18n string.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>