docs: improve onboarding and validate Python distributions in CI

[svalench]

Summary

• Add an early, cost-labelled quickstart, portable screenshot links, project badges, and contributor instructions.
• Classify the authored documentation site as documentation, not vendored code.
• Add the package Issues URL and PR-time wheel-from-sdist validation, including installed Django resources and all migrations.

Verification

• Python 3.12 / Django 5.2: 166 tests passed; coverage 86.77%.
• Ruff and mypy passed; documented focused tests: 4 passed.
• Build, strict Twine, isolated installed-wheel checks, and diff checks passed.
• Independent review found no blocking issues.

No runtime API changes, package publication, or invented adoption/benchmark figures. Other Python/Django combinations await GitHub CI.