chore: add page documenting use of alternative model providers

docs/docs/04-alternative-model-providers.md

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+# Alternative Model Providers
+
+
+## Usage
+
+GPTScript can be used against alternative models that expose an OpenAI compatible API or have a provider shim available.
+
+To use a model with an OpenAI compatible API:
+
+```gptscript
+model: mistral-large-latest from https://api.mistral.ai/v1
+
+Say hello world
+```
+
+To use a model that requires a provider shim:
+
+```gptscript
+model: claude-3-haiku-20240307 from github.com/gptscript-ai/anthropic-provider
+
+Say hello world
+```
+
+### Authentication
+
+For OpenAI compatible providers, GPTScript will look for an API key to be configured with the prefix `GPTSCRIPT_PROVIDER_`, the base domain converted to environment variable format, and a suffix of `_API_KEY`.
+As an example if you are using `mistral-large-latest from https://api.mistral.ai/v1`, the environment variable would be `GPTSCRIPT_PROVIDER_API_MISTRAL_AI_API_KEY`
+
+Each provider shim has different requirements for authentication. Please check the readme for the provider you are trying to use.
+
+## Available Model Providers
+
+The following shims are currently available:
+* [github.com/gptscript-ai/azure-provider](https://github.com/gptscript-ai/azure-provider)
+* [github.com/gptscript-ai/anthropic-provider](https://github.com/gptscript-ai/anthropic-provider)
+* [github.com/gptscript-ai/google-provider](https://github.com/gptscript-ai/google-provider)
+
+## Listing available models
+
+For any provider that supports listing models, you can use this command:
+
+```bash
+# With a shim
+gptscript --list-models github.com/gptscript-ai/anthropic-provider
+
+# To OpenAI compatible endpoint
+gptscript --list-models https://api.mistral.ai/v1
+```
+
+
+## Compatibility
+
+While the shims provide support for using GPTScript with other models, the effectiveness of using a
+different model will depend on a combination of prompt engineering and the quality of the model. You may need to change
+wording or add more description if you are not getting the results you want. In some cases, the model might not be
+capable of intelligently handling the complex function calls.