[codex] Fix GitHub Pages notebook rendering

[B67687]

What changed

This PR adds a GitHub Pages build pipeline that renders every Jupyter notebook into HTML before deployment.

Why it changed

The current GitHub Pages site renders README.md for the homepage, but all notebook links still point to raw .ipynb files. On Pages those files are served as notebook JSON, which makes the browser view look broken instead of showing a readable notebook page.

User impact

Notebook links on the published site will open rendered HTML pages instead of raw notebook JSON. The homepage keeps the existing README-driven structure, but its links are rewritten for the generated site.

Root cause

GitHub Pages does not automatically render linked .ipynb files from a repository site. The site needed a build step that converts notebooks to static HTML and deploys that output.

Validation

I could syntax-check the build script locally, but this desktop environment had local permission and nbconvert template issues that prevented a full local Pages build. The PR includes a clean GitHub Actions workflow so the build runs on a fresh GitHub runner instead.

[B67687]

Superseded by a replacement PR on branch pages/notebook-html-rendering: #2. I kept the same code changes there so the branch name is more descriptive.