Move code examples to rustdoc with compilation

[dannywillems]

Description

Code examples on the documentation website (https://leakix.github.io/oxide-sql/) should be:

1. Stored in rustdoc comments in the source code
2. Compiled during doc generation to ensure they're always valid
3. Referenced from the website, not duplicated

Current State

• Code examples are duplicated in markdown files
• Examples may become stale if API changes

Desired State

• Code lives in rustdoc comments with /// or //!
• Examples use # to hide boilerplate
• CI compiles all doc examples (cargo test --doc)
• Website pages link to rustdoc for examples

References

• Similar to how o1-labs/mina-rust handles documentation
• Docusaurus can redirect to docs.rs pages

Tasks

• Move examples from doc/docs/schema/queries.md to rustdoc
• Ensure all examples compile with cargo test --doc
• Update website to link to rustdoc instead of inline code
• Add CI step to test doc examples