[Providers] Establish a naming convention for endpoint pages

[Chesars]

Summary

The Providers page (https://models.litellm.ai/providers) has inconsistent endpoint naming:

• "Chat Completions API" —
• "Responses API (OpenAI Format)" — format in parentheses
• "OpenAI Images Generations API" — provider as prefix

Suggestion

Use {Endpoint} API ({Format}) consistently:

/chat/completions    → "Chat Completions API (OpenAI-compatible)"
/responses           → "Responses API (OpenAI-compatible)"
/messages            → "Messages API (Anthropic-compatible)"
/images/generations  → "Images Generations API (OpenAI-compatible)"

This clarifies what format to use without implying a specific provider destination.