DOC: <Issue related to /prompt_engineering/concepts>

[parkdg13]

Chat vs Completion
There are two different types of prompts: chat style prompts and completion style prompts.

Chat style prompts are a list of messages. This is the prompting style supported by most model APIs these days, and so this should generally be preferred.

Completion style prompts are just a string. This is an older style of prompting, and so mostly exists for legacy reasons.

The prompt type is written as chat and completion, but in the actual app, it is chat and instruct. Please explain the difference between completion and instruct and if there are any changes.