AI Gateway: Replace hardcoded version tags with variables for AI Prompt Compressor and PII Sanitizer plugin images

AI PII Sanitizer and AI Compressor docs have hardcoded Docker image version tags. Documentation shows `v0.1.2` when `v0.1.4` has been available for some time now. We need to add version variables to `jekyll.yaml` and reference them in the docs.

Affected pages:
- https://developer.konghq.com/plugins/ai-sanitizer/#access-the-docker-images
- https://developer.konghq.com/plugins/ai-prompt-compressor/#ai-prompt-compression-service

Start by reading jekyll.yaml, then inspect the Docker image references in the AI Sanitizer and AI Prompt Compressor pages listed in the issue. Add version variables there and update both documentation sections to reference them, with the current image versions shown instead of hardcoded outdated tags.

Written by the indexing model from the issue text.