Add reproducible build pipeline, rendering scripts and GitHub Pages workflow

[CybotTM]

Motivation

• Introduce a reproducible, source-driven build for the CV to separate editable source in src/ from generated public artifacts in public/ and enable GitHub Pages deployment.
• Provide a fallback rendering path when system tools are unavailable and standardize assets location to assets/ for consistent CSS usage.
• Automate build and deployment on pushes to main and enable checks that public artifacts are committed rather than generated ad-hoc.

Description

• Add a Makefile that defines build, html, pdf, index, check and clean targets and supports pandoc/weasyprint with Python fallbacks.
• Add simple rendering scripts scripts/render_html.py and scripts/render_pdf.py used when pandoc/weasyprint are not present.
• Add GitHub Actions workflow .github/workflows/build.yml to run the build on push/PR and upload to GitHub Pages for main branch deployments.
• Restructure repository: move CSS to assets/style.css, add src/ markdown sources (cv-executive.de.md, cv-technical.de.md), update README.md, adjust .gitignore, and add generated public/ HTML/PDF artifacts while removing the old single-file source and HTML.

Testing

• Ran make build which produced public/cv-executive.de.{html,pdf}, public/cv-technical.de.{html,pdf} and public/index.html successfully.
• Ran make check (which executes git diff --exit-code public/) and the check completed successfully.
• The new GitHub Actions workflow was added to trigger automated builds and Pages deployment on main, and is ready to run on the next push.

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Codex Task