Write CONTRIBUTING.md for publications workflow

[nlaverdure]

This is the highest-priority "student-facing" piece.

• Write CONTRIBUTING.md at a high-school reading level covering step by step:
  1. Cloning the repo / opening it in an editor.
  2. Running the site locally (npm install, npm run dev).
  3. Adding a new publication: create src/content/publications/<slug>/, copy
     _template.md to index.md, fill in frontmatter, drop image file alongside it.
  4. Editing existing page text — page prose lives in .astro files; sponsor data
     lives in src/pages/partners/sponsors.json.
  5. Committing changes and opening a pull request.
  6. What CI checks mean and how to fix common failures (format errors).
• Template file already exists at src/content/publications/_template.md.