docs: migrate guides to wiki and slim README

[ngan]

Summary

This migrates FixtureKit documentation to the GitHub Wiki and keeps the repository README focused on quick start onboarding.

What changed

• Rewrote README.md as quick start only.
• Added canonical API/reference doc at docs/reference.md.
• Published task-based wiki pages + sidebar navigation.

Wiki

• Root: https://github.com/Gusto/fixture_kit/wiki
• Home: https://github.com/Gusto/fixture_kit/wiki/Home

Wiki page index

1. Home
2. Getting Started
3. Declaring Fixtures
4. Using Fixtures in RSpec
5. Using Fixtures in Minitest
6. Configuration and Adapters
7. Cache Lifecycle and Performance
8. Callback Hooks
9. Anonymous Fixtures and Identifiers
10. Multi-Database Behavior
11. Troubleshooting

Notes for reviewers

• No runtime code changes.
• Existing deep README anchor links are intentionally not preserved.
• API-level signatures/defaults are now canonical in docs/reference.md; wiki pages link to this reference for exact contracts.