docs: streamline README and reorganize v1.1 documentation (#306)

[Mattsface]

Why

The README has grown into a mix of quick-start documentation, transport reference material, migration guidance, endpoint documentation, and release history.

For v1.1.0, the goal is to make the README easier to scan for new users while preserving the deeper technical documentation in dedicated pages.

This also adds clear documentation for the new async client without turning the README into another full API reference.

Tracks #306.

What

• Significantly shortened and reorganized the README
• Added sync and async installation and quick-start examples
• Added a sync vs async comparison and concurrency example
• Added docs/async.md for detailed AsyncMlb usage
• Added docs/examples.md for longer usage examples
• Restored the method documentation previously removed from the README in docs/methods.md
• Linked the method reference from the README
• Moved detailed transport and API contract information behind links to their authoritative documentation
• Updated CONTRIBUTING documentation and development/test instructions
• Preserved the existing public API and transport documentation rather than duplicating it in the README

Tests

This PR contains documentation changes only.

The documentation was reviewed against the current release/1.1.0 API and async contract. Code examples and documented method names were checked against the current public API documentation.

Normal CI should still run before merge.

Risk and impact

Risk: Minimal

There are no production code or public API changes in this PR.

The main risk is documentation becoming inaccurate or linking users to the wrong guidance, particularly while async endpoint coverage is still being finalized.

If something does go wrong, the impact should be limited to confusing or incorrect documentation. It would not change runtime behavior for existing users.