docs: add GitHub Actions CI workflow documentation

Document the GitHub Actions infrastructure for kdevops CI, including the
manual guest OS selection feature and reusable action patterns for external
projects.

The documentation covers the two main workflows (kdevops.yml for full testing
and config-tests.yml for validation), explains the modular composite action
architecture, and provides examples for other repositories to leverage kdevops
CI infrastructure without duplication.

Key sections include guest OS selection usage (default, debian-13, fedora-41),
external project integration patterns with version pinning strategies, complete
workflow reference, and troubleshooting guidance.

Generated-by: Claude AI
Signed-off-by: Daniel Gomez <[email protected]>

Author: dkruces