Migrate to Nx monorepo with create-eslint-config package

[hanna-skryl]

Closes #46

Summary

• Convert repository to Nx monorepo with two packages: @code-pushup/eslint-config (existing) and @code-pushup/create-eslint-config (new, coming soon)
• Configure Nx to orchestrate tasks (test, lint, typecheck, build, docs) with caching
• Replace release-it with Nx releases
• Update CI pipeline to use nx affected for checking affected projects
• Set up Commitlint and Commitizen for conventional commit enforcement
• Add monorepo documentation (root README, CONTRIBUTING.md)

Note

The npx nx g @nx/workspace:convert-to-monorepo generator did not produce a usable result for this repository (mangled package name, incomplete file movement, nested scope directory). The monorepo structure was created manually instead.