Create section for deploying Spice through CI/CD

[lukekim]

[claudespice]

Triage notes:

Current state of website/docs/deployment/: aws/, architectures/, cloud/, docker/, kubernetes/, plus index.md. No CI/CD section exists.

This is a sizeable new content addition rather than a content fix — would benefit from scope guidance before drafting:

• Which CI/CD systems should be covered? (GitHub Actions, GitLab CI, Argo CD/Flux for K8s GitOps, Azure DevOps?)
• Target audience: Spicepod authors deploying through CI vs. teams running Spice as a sidecar/sidekick built into existing pipelines?
• Scope of "deploy through CI/CD" — building Spicepod images, rolling K8s manifests, running spice install/spice add in pipelines, secrets handling?

Leaving open for that scope input. Once the surface is decided, the page would slot under website/docs/deployment/ci-cd/.

[lukekim]

For Spice Cloud use https://github.com/spicehq/spice-cloud-deploy-action.

[claudespice]

Picked this back up after @lukekim's pointer to spicehq/spice-cloud-deploy-action.

Opened #1651 with a starter Deployment > CI/CD page covering:

• The spice-cloud-deploy-action GitHub Action (minimal workflow, common inputs, prerequisites).
• Pointers to the existing Argo CD / Flux GitOps guides for self-hosted Kubernetes.
• A note that AWS / Azure / Docker pipelines invoke the existing provider guides.

Leaving the issue open so deeper coverage (GitLab CI, Jenkins, Azure DevOps, Spice runtime CI rather than Spicepod deploys) can land as follow-ups if you want them.