[AIG] Initial documentation for prompt management

[daisyfaithauma]

Summary

• Initial documentation for prompt management

Screenshots (optional)

Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[github-actions]

Preview URL: https://a512e69d-cloudflare-docs.cloudflare-docs.workers.dev

[kathayl]

• let's have prompt management be at the same level of Evaluations. don't think it fits under Configuration and in the future we will add more subpages to it. ex: a/b testing prompts, prompt playground
• Similar to Evaluations, let's split this existing prompt management page into two

1. overview where we give some context about why prompt managemet is importnat and what our feature does. I posted copy: https://docs.google.com/document/d/1ZXK730DNxVSoi_YO0YkFCHb_C5YS7My-fBTgXdeUSVU/edit?tab=t.0
2. Set up prompts (which you have already written)

I'm going to hold on reviewing the steps for creating and managing prompts because there will be some slight changes to design, that we reviewed with Arun today. So let's wait until the updated preview in case there are steps we'll need to update