docs: remove fixture scoping design document and simplify README (#85)

- Deleted `fixture-scoping-design.md`, as it is no longer relevant.
- Updated README:
  - Consolidated sections on async, parallelism, and smart tagging for improved readability.
  - Removed redundant examples, outdated parameterization details, and fixture scoping explanations.
  - Highlighted key features and added a link to full documentation.

Author: renaudcepre