Docs: add fixture-based offline testing guidance and expand Hacktoberfest onboarding

[PUSHPAK-JAISWAL]

Summary

• document offline fixture usage with --fixture, CPA_CATALOG_FIXTURE, and CPA_FIXTURE_DIR

• add copy-pasteable examples for listing templates and running a smoke scaffold offline

• expand the Hacktoberfest guide with repo suggestions, labels, and guidance for avoiding duplicate work on assigned epics

Test plan

1. uv run create-awesome-python-app --help
2. uv run create-awesome-python-app --fixture . --list-templates
3. uv run create-awesome-python-app --fixture . --template create-python-app/fixtures/templates/example-cli --no-install --no-interactive .\tmp-cpa-smoke

Closes#249
Closes#248
Closes#247