Update Users section of the docs

[andrii-i]

Problem

1. Currently "Model providers" section of the user docs (https://jupyter-ai.readthedocs.io/en/latest/users/index.html#model-providers) shows an example of setting an API key as env variable. But that will only work for the magics providers. To set API keys for chat interface, users need to use chat settings UI or config.json.

Note that users are getting introduced to %ai magic commands in "The %ai and %%ai magic commands" section of the docs (https://jupyter-ai.readthedocs.io/en/latest/users/index.html#the-ai-and-ai-magic-commands) which is located much further down the page.

2. The chat interface section of the user docs has screenshots of previous version of chat settings UI https://jupyter-ai.readthedocs.io/en/latest/users/index.html#the-chat-interface

3. Currently user docs have “Custom model providers” and “Customizing prompt templates” sub-subsections that explain how to customize J-AI with code before “The chat interface” subsection that explains how to use J-AI's interface.

All three might be confusing for new users not familiar with Jupyter-AI and make onboarding experience hard(er).

Proposed Solution

1. Update "Model providers" section of the docs replacing use of env variable in the example with a use of chat settings UI or config.json.
2. Update chat settings screenshots
3. Promote “Custom model providers” and “Customizing prompt templates” sub-subsections to sub-sections, move them to the bottom of the users doc page, below “The chat interface” and “Configuration” sub-sections section. Now table of content would look like:

[andrii-i]

@JasonWeill @3coins @krassowski thank you very much for looking into this PR. All comments were addressed, please give it a look again when possible.

[JasonWeill]

The screen shot doesn't show any OpenAI language models. While we as users know that we can scroll down to pick an OpenAI model, in the docs, it might be better to show a screen shot with an OpenAI model visible.

Alternatively, you can change the text above to say, "For example, to use AI21 models, …"

[3coins]

LGTM! Thanks for making all the updates.

[andrii-i]

Thank you for looking into this everyone.