docs: add contributor guide for variant pages

[hanthor]

Summary

• add a step-by-step guide for creating a TunaOS variant reference page
• document build-matrix, sidebar, GHCR, ISO/R2, and generated screenshot conventions
• link the guide from the repository CONTRIBUTING.md and the docs sidebar

Why

New contributors currently have to reverse-engineer variant pages and their generated data sources. This guide gives them a copyable skeleton, exact navigation and image rules, and a PR checklist.

Validation

• git diff HEAD^ --check
• local npm run typecheck / npm run build were not run because this checkout has no node_modules; CI should provide the project dependencies

Closes #231

[hanthor]

Triage note — keeping this one. It is not superseded: there is no docs/variant-pages.md on main, and CONTRIBUTING.md has no "Adding a variant page" section.

The content holds up on review — it points at tunaOS/.github/build-config.yml as the source of truth for flavors and image tags, tells contributors not to hand-edit static/iso-index.json or the generated screenshot gallery, and warns against claiming an ISO for a container-only flavor. Those are exactly the failure modes that produce wrong install instructions, which is the same class of problem #347 was about (now gated by the checker merged in #362).

Blocked only on process:

1. Still a draft — needs to be marked ready for review.
2. Needs a rebase onto current main before merge; sidebars.ts has moved since 2026-08-15.

I did not rebase it because the PR head is on a fork this session cannot push to. Related issue: #231.

---

Generated by Claude Code