docs: explain Python serverless environments

[TangoEnSkai]

Resolves #1055

Description

Document how Python models use environment_key and environment_dependencies with serverless compute, including how to provide a complete Jobs API environment through python_job_config.environments. Add the guide to the repository documentation indexes.

Validation

• pytest tests/unit/python/test_python_config.py tests/unit/python/test_python_job_support.py -q (41 passed)
• git diff --check
• Databricks integration tests were not run because this is a documentation-only change.

Checklist

• 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
• I have updated the CHANGELOG.md and added information about my change to the "dbt-databricks next" section.