docs(evaluation): rewrite custom models guide to house style

[abhijaisrivastava15]

Summary

Rewrites the "Use custom models" guide to match the house style used across the rest of the Evaluation batch:

• Drops the <Steps> wizard layout and generic per-provider placeholder images, one H2/H3 per action instead
• Fixes the nav path to the real one: Settings -> AI Providers -> Custom model tab -> Create custom model
• Real drawer copy (Add Model, From model Provider / Configure Custom Model) instead of the old generic "model configuration (e.g. Settings or Models)" phrasing
• 4 live screenshots: the Custom model list, the provider dropdown (OpenAI/Bedrock/SageMaker/Vertex AI/Azure), the OpenAI form, and the Configure Custom Model form
• States clearly that saving IS the connectivity check (no separate test-connection step), matching what the UI actually does

Test plan

• Astro dev renders the page with no MDX errors
• All 4 images load and match current product UI
• Links resolve: /docs/evaluation/guides/custom-evals, /docs/evaluation/guides/running-evaluations, /docs/evaluation/concepts/evaluator-models, /docs/evaluation/guides/cicd, /docs/evaluation