docs: Add integration test guides to adapter CONTRIBUTING.md files

[pangyihahn]

resolves #843

Problem

The issue #843 noted that the CONTRIBUTING.md file for nearly all adapters lacked the necessary documentation detailing how to run integration tests locally. The files only contained boilerplate text, forcing contributors to manually investigate environment variables and run commands.

Solution

This PR creates comprehensive CONTRIBUTING.md files for dbt-athena, dbt-bigquery, dbt-postgres, dbt-redshift, and dbt-snowflake. Each file now clearly outlines the required environment variables (from their respective test.env.example files) and provides specific hatch run integration-tests commands for running the full test suite or single test classes.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

[pangyihahn]

Hi may I have this checked and merged?