Add .nojekyll so GitHub Pages serves underscore-prefixed dirs

Without this, Jekyll strips _playground/, _framework/, _content/ etc.
when the site is served from gh-pages. The playground was brand-new
in the last deploy so this only surfaced once it actually shipped.

docs/public/ is copied verbatim into docs/.vitepress/dist/ by VitePress,
so a single .nojekyll file there lands at the dist root.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: koenbeuk