ci: add renovate action

[a-klos]

This pull request introduces extensive updates to the repository, primarily focusing on enabling and configuring Renovate for dependency management and simplifying image repository structures across the infrastructure files. Key changes include adding Renovate configuration and workflow files, updating image repository paths, and streamlining Helm template logic.

Renovate Integration:

• Added .github/renovate.json to configure Renovate with rules for dependency updates, grouping, automerging, and labeling. Custom managers and host rules for Helm charts and Docker images were also defined.
• Added .github/workflows/renovate.yml to schedule and manually trigger Renovate runs using a GitHub Actions workflow.

Image Repository Updates:

• Updated image repository paths in infrastructure/rag/values.yaml for backend, frontend, adminBackend, adminFrontend, and extractor to use more specific paths (e.g., ghcr.io/stackitcloud/rag-template/backend). [1] [2] [3] [4] [5]
• Updated the backend image repository in infrastructure/README.md to match the new path.

Helm Template Simplification:

• Simplified Helm templates (_admin_backend_and_extractor_helpers.tpl, _admin_frontend_helpers.tpl, _backend_helpers.tpl, _frontend_helpers.tpl) by removing the name field from image definitions and adjusting the printf logic to construct the image name directly from repository and tag. [1] [2] [3] [4]