Add Jekyll site structure, navigation, and GitHub Pages deploy workflow

[nduytg]

Motivation

• Convert the repository to a GitHub Pages-friendly Jekyll site with clear collections and navigation to make content easier to publish and browse.
• Provide a reusable site layout and content entry points so evergreen cheat sheets, projects, and time-based blog posts are presented consistently.
• Enable automated build and deployment to GitHub Pages and remove the custom CNAME so the repository uses the default Pages domain pattern.

Description

• Add a GitHub Actions workflow (.github/workflows/pages-deploy.yml) to build the site with Jekyll, upload the generated _site, and deploy Pages on pushes to main.
• Update _config.yml to define site metadata, docs and projects collections, and default layouts, and add _data/navigation.yml for top navigation.
• Add a base layout (_layouts/default.html), collection content (_docs/getting-started.md, _projects/platform-engineering-case-study.md), a new blog post (_posts/2026-04-19-site-structure-update.md), and site pages (index.md, cheat-sheets.md, projects.md, blog.md, about.md).
• Update README.md to document the Jekyll content model and CI/CD workflows, remove CNAME, and tweak pymarkdown config (.pymarkdown.json) to enable front-matter and adjust rules.

Testing

• Ran local pre-commit hooks and pymarkdown linter via pre-commit run --all-files, and the checks completed successfully.
• The new GitHub Actions Pages workflow is configured to build on pull requests and deploy on pushes to main, and it will run on the next eligible push to main (no deployment run occurred in this PR).

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Codex Task