GenAI: Add system-specific conventions

[lmolkova]

We have a pattern in other conventions where we define generic conventions along with system-specific ones.
System-specific conventions describe how generic apply to them (e.g. we'd want to update some attribute descriptions for Cohere or Anthropic) and may define additional attributes.

This issue tracks adding docs for all known gen_ai.system (but it should probably be broken down into individual PRs)

• OpenAI
• Cohere
• Anthropic
• Vertex AI
• Azure AI Inference is coming soon