docs: Comprehensive rewrite of project documentation

[miguemagicdev]

Description

This PR replaces the placeholder documentation with a formal technical specification for the PhysContinuum Engine. The goal is to clearly communicate the engine's focus on scientific accuracy over real-time performance.

Changes

• Project Identity: Rebranded the description to emphasize non-real-time, high-fidelity simulation.
• Technical Roadmap: Defined core objectives including numerical precision and temporal stability.
• Status Transparency: Added a "Work in Progress" section to clarify that the current codebase is in an architectural phase.
• CI Visibility: Integrated badges to track the existing status-checks.yml workflow and MSRV (Minimum Supported Rust Version).

Context

The previous README was insufficient for an open-source scientific project. This update ensures that anyone visiting the repository immediately understands the project's rigorous standards and intended use cases.

Verification

• All Markdown headers follow a consistent hierarchy.
• Badge URLs are mapped to the existing Magirenko/physcontinuum repository.