docs: add Tiyuvta custom provider guide

[avifenesh]

Summary

• add a focused Tiyuvta custom-provider page with the OpenAI-compatible base URL and API-key setup
• document the public model-discovery request, custom-model registration, and a logged PromptLayer SDK smoke test
• add the page under Providers → Custom Providers

Live verification

• https://inference.tiyuvta.ai/login?next=/app/keys returns HTTP 200
• https://api.tiyuvta.ai/v1/models returns the current public model catalog without authentication
• PromptLayer’s current Python SDK passes a custom provider’s stored API key and base URL to the selected client

Validation

• mint validate
• jq empty docs.json
• git diff --check

No PromptLayer SDK or API behavior changes are required.