Update destination_name documentation to refer to destination slug

What does this MR do and why?

To avoid confusion, this updates the API documentation of bulk imports to make clear that 'destination_name' actually refers to the destination slug.

I've used "destination slug" instead of "destination path" because slug refers to the last section of a URL, whereas path refers to the entire URL.

We will eventually update the backend to change 'destination_name' to 'destination_slug', but thought that updating the documentation now was a good initial step.

Screenshots or screen recordings

Improved documentation

How to set up and validate locally

Visit http://gdk.test:3000/help/api/bulk_imports.html#start-a-new-gitlab-migration on a local instance run from the branch

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #363390 (closed)