Update to suggest defineDotPrompt rather than definePrompt

[i2amsam]

Update prompts.md to suggest defineDotPrompt rather than definePrompt

Checklist (if applicable):

• [ ] Tested (manually, unit tested, etc.)
• [ ] Changelog updated
• [ ] Docs updated

[MichaelDoyle]

I made a bunch of comments in my "review", but I also want to make the following suggestion:

I think we should combine this page and the dotprompt page into one. The two pages currently overlap, and are in conflict with each other in a few different ways.

For example, this page suggests developers should start with defining prompts in code (defineDotPrompt) and yet, the dotprompt documentation suggests that developers should start with the file format first, and introduces defineDotPrompt is an alternative method if you really want to write it as code instead.

Both are covering input/out schemas, but in varying levels of detail, etc.