docs: enhance README with repository structure and key features

[genius-0963]

Description

This pull request enhances the project's root README.md to provide a more comprehensive, professional, and accessible introduction to the MuJoCo physics engine.

As the project scales and attracts more contributors and users, it is crucial that the primary entry point clearly outlines the repository's structure and the core capabilities of the software.

Key Changes

1. Key Features Section: Introduced a concise, bulleted list highlighting MuJoCo's fundamental strengths, including its generalized coordinate formulation, continuous-time contact dynamics, and native optimization support. This immediately answers "Why MuJoCo?" for new researchers and developers.
2. Repository Structure: Added a directory-level breakdown detailing the purpose of major folders (src/, python/, mjx/, plugin/, simulate/, etc.). This greatly reduces friction for new contributors trying to navigate the codebase.
3. Consolidated Community Guidelines: Refactored the "Contributing," "Asking Questions," and "Bug reports" sections into a unified Community & Contributing section. This improves flow and makes the contribution pathways clearer.

These documentation updates do not alter any functionality but significantly improve the developer experience (DX) and project presentation.

Type of Change

• 📚 Documentation Update (adding details to README.md)

Checklist

• I have read the CONTRIBUTING document.
• My changes do not introduce any new warnings or errors.
• The formatting aligns with the project's STYLEGUIDE.