feat(ai-gateway): Missing Cerebras provider documentation

[tomek-labuk]

Missing Cerebras provider documentation

We need to fill in the gaps in the AI Providers landing page. We're missing how-to guides for setting up AI Proxy and AI Proxy Advanced for Cerebras chat routes.

DoD:

• How-to for using Cerebras with AI Proxy similar to this guide
• How-to for using Cerebras with AI Proxy Advanced similar to this guide
• Cerebras section on the AI Providers page updated with a how-to block:
  • Make sure all provider tags are in the allowed tags list. If not, add them.

  - header:
      type: h4
      text: How-to Guides
    columns:
      - blocks:
          - type: how_to_list
            config:
              plugins:
                - ai-proxy
                - ai-proxy-advanced
              tags:
                - cerebras
              quantity: 3
              allow_empty: true