docs: describe the PostgreSQL contract and supported types

[omer-cengel]

Summary

sqlcj targets PostgreSQL and JDBC, but nothing in the repository stated that
contract or which SQL types and CREATE TABLE constructs a schema snapshot may
actually use. This adds a single user-facing document that states the database
contract of configuration version "1" and publishes the supported type and DDL
surface, describing only behavior the existing tests demonstrate.

Changes

• Document configuration version "1" as PostgreSQL schema and query input with
  blocking JDBC execution, with no engine option and no dialect option.
• State that the application owns the connection: the runtime is constructed
  with a javax.sql.DataSource, no JDBC driver is shipped, and behavior is
  verified against PostgreSQL 16 while H2 remains a test-only convenience.
• Publish the accepted column types and alias spellings with their Java
  mappings, including case-insensitive matching with type arguments ignored.
• Separate the accepted, ignored, and rejected CREATE TABLE constructs, and
  record that constraints never change generated Java.
• Describe the null contract: reference types throughout, uniform positional
  binding and reading, and NOT NULL as the only source of column nullability.
• List the rejected type spellings and unsupported type families, the CLI
  failure behavior, and the tests that back each published table.

Scope and non-goals

• Documentation only. No production, test, or build file changes, and no
  documentation-verification machinery such as Markdown-parsing tests.
• No release packaging, README, quickstart, or distribution documentation.
• Query shape, cardinality, parameter ordering, naming, and runtime-ownership
  contracts are only described where the schema and type contract requires it.