Simplify README and link to rustdoc from website

[dannywillems]

Description

1. Remove Quick Start section from README - redirect to website/rustdoc
2. Link rustdoc from Docusaurus website

README Changes

• Keep only: project description, features list, crate overview
• Remove: Quick Start code examples (duplicate of website/rustdoc)
• Add prominent links to:
  • Website: https://leakix.github.io/oxide-sql/
  • Rustdoc: https://docs.rs/oxide-sql-core

Docusaurus Changes

• Add link to rustdoc in navbar
• Add "API Reference" section linking to docs.rs
• Schema/queries page should link to rustdoc examples

Benefits

• Single source of truth for code examples
• Less maintenance burden
• Examples always up-to-date (rustdoc compiles them)