Add guide page on how to add DEEPINFFRA API (see below for details)

I was looking for cheaper sites that give access to llama3-70b than openrouter, and found https://deepinfra.com.

To add this model as a custom modem in TM: https://api.deepinfra.com/v1/openai/chat/completions

Model ID: meta-llama/Meta-Llama-3-70B-Instruct

Supports openAI plugins

Custom header for API

Authorization : Bearer {insert your API}

Endpoint: https://api.deepinfra.com/v1/openai/chat/completions