docs: refresh project documentation

[adrianogil]

Summary

• rewrite the English README to describe the current mesh generators, processing tools, scenes, shaders, and Unity setup
• add an independently written Spanish README with reciprocal language navigation
• add contribution guidance for responsible automated and AI-assisted submissions

Why

The previous README was too generic and referenced an incomplete repository layout. The refreshed documentation gives readers accurate entry points into the current experiments and establishes how automated contributions are disclosed and assessed.

Impact

Documentation only. No Unity assets, scripts, packages, project settings, or runtime behavior are changed.

Validation

• verified every documented scene and project path against the repository
• confirmed the English and Spanish READMEs link to each other
• ran git diff --check
• confirmed the branch contains only the three intended documentation files