docs: add detailed section on sync module and its integration with loom

[soltanoff]

This pull request adds documentation to clarify the purpose and usage of concurrency testing tools and the custom sync module in the project. The most important changes include new explanations and links for loom and Miri in the main README.md, and a detailed guide for the sync module in its own README.

Concurrency testing tools documentation:

• Added a brief description and link to the loom library in README.md, explaining its role in testing parallel and concurrent programs.
• Added a brief description and link to the Miri interpreter in README.md, outlining its ability to detect undefined behavior in Rust code.

sync module documentation:

• Created a new src/sync/README.md file explaining the motivation, design, and usage of the sync abstraction layer for compatibility with loom, including how it enables deterministic concurrency testing and how to use it in the project.