insights: Add docs for Managing Code Insights with the API

Created by: CristinaBirkel

Closes https://github.com/sourcegraph/sourcegraph/issues/28519

Description

This is a how-to guide for common API use cases, including CRUD operations on insights, as well as how to create a dashboard and add insights to it.

Testing Steps

I ran through all of the examples and tried them out in our API Console to verify that they worked!