build: switch inspektor-gadget fork from matthyx to kubescape (#925)

The personal fork (matthyx/inspektor-gadget) has been migrated to
kubescape/inspektor-gadget so the dependency isn't tied to one
contributor's account. Repoint the go.mod replace directive and
document the fork in docs/features/inspektor-gadget-fork.md.


Claude-Session: https://claude.ai/code/session_01Byg9gGPP9bgXhVnPAqKFro

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>