docs(pgsql-client): add ephemeral database documentation

[pyramation]

Summary

Adds documentation for the createEphemeralDb function to the pgsql-client README. This function creates temporary PostgreSQL databases with unique UUID-based names, useful for testing, code generation, and CI pipelines.

The documentation covers:

• Basic usage example
• Configuration options (prefix, extensions, baseConfig, verbose)
• Return value description (name, config, admin, teardown)
• Common use cases

Review & Testing Checklist for Human

• Verify API accuracy — Compare the documented API against postgres/pgsql-client/src/ephemeral.ts to ensure options and return values match the implementation
• Test the example — Run the basic usage example to confirm it works as documented

Notes

Link to Devin run: https://app.devin.ai/sessions/5edec0c962d04ee9bd15efc58ca2a050
Requested by: @pyramation

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