Documentation on Customized Model Service

[Wenzhi-Ding]

Currently, edsl covers mainstream models. However, researchers may be interested in using some customized model services. I just explored a way to add support for any model services. Maybe some examples to be included in the documentation can help other scholars customize it more easily.

This customization includes:

1. Create a new CustomService class
2. Modify the API URL, API token/key, request header, body, and response parser
3. Store API token/key in .env
4. Modify enum.py and registry.py to cover the customized model

I can create that documentation and PR if this is a good idea.