docs: document `skip_guest_setup` option via API and CLI

Document the skip_guest_setup pipeline setting that allows users to skip pre-artifact and post-artifact installation tasks (guest setup) on the provisioned test environment.

Added a new section in test-request.adoc showing both the API payload (.environments[].settings.pipeline.skip_guest_setup: true) and the CLI flag (--skip-guest-setup), with a cross-reference from the test-process.adoc guest setup section.

Assisted-by: Claude Code Signed-off-by: Miroslav Vadkerti mvadkert@redhat.com