Improve contributor onboarding and consolidate contribution documentation

The contribution documentation was previously duplicated across multiple locations (README.md, CONTRIBUTING.md, and site/src/templates/contribute.html), which made maintenance difficult and caused sections to drift out of sync.

This issue tracks the consolidation of all contribution guidance into CONTRIBUTING.md as the single source of truth, with the website contribution page rendered directly from it at build time.

The changes also improve onboarding for new contributors by:

clarifying the contribution workflow,
documenting folder/category selection,
explaining how to mark problems as solved,
adding Lean 4 setup guidance,
centralizing references and style conventions.

Additionally, external documentation links on the website now open in a new tab to improve navigation during the contribution process.

Implemented in: #3967