docs(contributing): document fixtures/ offline catalog testing

[ulises-jeremias]

Context

The repo has fixtures/ and CLI flags/env vars for offline catalog testing (--fixture, CPA_CATALOG_FIXTURE, CPA_FIXTURE_DIR), but CONTRIBUTING.md barely documents them. Newcomers struggle to run CLI tests without network.

Steps

1. Read how fixtures are used in packages/create-awesome-python-app/tests/ and CLI help.
2. Add a "Testing with fixtures" section to CONTRIBUTING.md modeled after CNA's fixture docs (adapted to CPA names).
3. Include example commands for --list-templates / a smoke scaffold using fixtures.
4. Link relevant tests.

Acceptance criteria

• Documents CPA_CATALOG_FIXTURE, CPA_FIXTURE_DIR, and --fixture
• Shows at least one copy-pasteable example
• Describes fixture directory layout at a high level
• Links to the tests that exercise fixtures

Files

• CONTRIBUTING.md
• fixtures/ (reference)
• CLI / tests under packages/create-awesome-python-app/

How to verify

A new contributor can follow the section and list templates offline without hitting GitHub raw URLs.