Remove system prompt from playground creation page

[slemeur]

We would like to improve the experience on creating a playground and defining the system prompt.

First time user
We are asking the user to define something that is not yet known. The change it will do to the way the model interact will not be understood AND it will not be editable "on the go"

--> The system prompt should be treated as an advanced setting in the creation of the playground environment

Right now:

• The user does not what's mandatory from what's optional
• The user will be intrigued by the "system prompt", while it's not something needed for the first time experience -> adding more cognitive load

User interacting with the model
As a user I've interacting with the model and I'm writing a lot of messages. I will lost the system prompt and need to scroll to the top to see what was configured.
While the system prompt is a message in the conversation, it's also a setting that impacts the entire conversation. As a result, it would be easier if the user was able to see at any point in time in the conversation, without doing back and forth with the beginning of the conversation.

User iterating customizing the use of the model
As a user, I want to be able to adjust and iterate into the definition of how my model must interact. It's a multi-stage purposes.

• User will start without defining a system prompt
• User will adjust the settings and add a system prompt, replay the discussion to observe how it change the behavior of the model
• User will continue to adjust the system prompt and would like to be able to compare the behavior
  It's going to be an iterative process, but here, because of this UX, the user is forced to each time get back to the creation of the playground environment.

We would need to introduce a capability to edit the system prompt and let the user decide either:

• to create a new playground environment --> so the user has a fresh session where it's possible to start from blank
• to replay the current conversation but with a different system prompt.

That also means that there should be a way to "edit" the system prompt directly from the playground page.