Add GitHub Pages deployment with interactive Stan Playground support

[magland]

This PR introduces automatic deployment of these examples to GitHub Pages with interactive Stan Playground embeds. It adds a CI/CD pipeline, Jekyll configuration, and helper scripts to generate index.md files for browsing models interactively.

For a preview, I have deployed this on my fork here:
https://magland.github.io/example-models/

You can navigate the folders, and when there is a directory with .stan file(s), you will see those embedded in Stan Playground where you can edit/run the models. The system also looks for .data.json files adjacent to the .stan files and loads those into the SP window.

Changes:

• GitHub Actions Workflow: Added .github/workflows/ghp-deploy.yml to build and deploy site via GitHub Pages. Workflow generates index files, builds Jekyll site, and deploys only on main/master pushes
• Deployment Tools: New ghp-deployment/ directory with scripts to generate the index.md files embedding Stan Playground, etc.
• Jekyll/GitHub Pages Setup: Added Gemfile and _config.yml, updated .gitignore to exclude _site and Gemfile.lock
• Repository Documentation: Updated top-level README.md with link to the interactive browser
• Minor adjustments: needed to fix some Jekyll Latex formatting issues in a couple .Rmd files.

IMPORTANT: If this PR were to be accepted, we would need to modify a couple hardcoded strings. For example https://magland.github.io would need to become https://stan-dev.github.io and PAGES_REPO_NWO: magland/example-models would need to become PAGES_REPO_NWO: stan-dev/example-models

In order for this to work, the pages configuration for this repo would need to be turned on and set to GitHub Actions in Settings.

Worked on this with @WardBrian