Remove legacy Jekyll source files from gh-pages

After the Hugo migration (STF-448), the rendered site on gh-pages is
fully managed by `.github/workflows/pages.yml`. The Jekyll source files
that pre-dated the migration survived `keep_files: true` on the
workflow's initial run but are no longer used to render the site —
GitHub Pages now sees `.nojekyll` and serves the prebuilt Hugo output.

These files remain publicly fetchable today though, and `_layouts/default.html`
still references a CSS host this migration is retiring. Delete them to
prevent confusion and remove that lingering reference.

Preserves: `doc/` (versioned API docs), `doc/latest` (symlink), `.nojekyll`,
`index.html`, `css/`, and everything else pushed by the Hugo workflow.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: oschwald