ci: add GitHub Pages workflow for API docs

Deploy the Scalar API reference to GitHub Pages on pushes to main
when openapi.json or docs/api/ changes. Copies both files into a
flat _site directory for the Pages artifact.

Requires enabling GitHub Pages with "GitHub Actions" as the source
in the repository settings.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sachiniyer