docs: audit SQL module page

[demengc]

Summary

• Fixed batch.bind() -> batch.batch() bug (method name did not match BatchBuilder API)
• Simplified constructor examples to use null defaults instead of manually building the JDBC URL string
• Documented Sql.MYSQL_DRIVER, Sql.MYSQL_LEGACY_DRIVER, and Sql.DEFAULT_JDBC_URL constants in a reference table
• Added async operations section with sync/async method mapping table
• Replaced manual Schedulers.async() wrapping in examples with the built-in executeAsync/queryAsync methods
• Documented SqlCredentials.of(ConfigurationSection) factory method
• Documented low-level access methods (stream(), getConnection(), getHikari())
• Documented BatchBuilder.execute() and BatchBuilder.reset() methods
• Corrected cleanup section: Sql implements ISql which extends Terminable (not directly AutoCloseable)
• Used no-preparer overloads of execute(String) and query(String, SqlFunction) where appropriate
• Removed redundant inline comments and filler text throughout

Test plan

• Verify all code examples match the current API signatures in source
• Confirm markdown renders correctly on GitBook