Fix AI docs workflow by adding placeholder signature files

The workflow was failing because the Dockerfile expected signature
and certificate files (.sig and .crt) that weren't being created.
Added placeholder files to unblock the workflow until proper
signing is implemented in the next phase.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 (1M context) <[email protected]>

Author: ltagliaferri

.github/workflows/compile-ai-docs-from-gcs.yaml

@@ -156,6 +156,11 @@ jobs:
         # Copy files for container build
         cp static/downloads/chainguard-ai-docs.md scripts/
         cp static/downloads/checksums.txt scripts/
+
+        # Create placeholder signature files if they don't exist
+        # (These will be properly signed in a future phase)
+        touch scripts/chainguard-ai-docs.md.sig
+        touch scripts/chainguard-ai-docs.md.crt
         
         # Build container with GitHub Container Registry
         docker build -f scripts/Dockerfile.ai-docs -t ghcr.io/${{ github.repository_owner }}/ai-docs:latest scripts/