Merge / de-duplicate instructor checkout content

### How could the content be improved?

These two pages on the checkout process have some duplicated content and look very similar.

https://carpentries.github.io/instructor-training/instructor/14-checkout.html
https://carpentries.github.io/instructor-training/instructor/checkout.html

(Long anecdote - During a recent training, I got very confused because my co-instructor shared the second page, and when I went to go open the lesson, I found the first page via the sidebar table of contents, and 1) did not immediately realize these were different pages; and 2) could not figure out why "my" version of the page lacked the links to details on each of the three checkout steps. The fact that the second page is by default hidden in the sidebar did not help).

Possible solutions:

- merge the two pages into once single page (although I note that the second page is linked from the Handbook, https://docs.carpentries.org/handbooks/instructors.html#onboarding)
- remove duplicated content so that each page has only unique content
- move the detailed checkout information to the handbook rather than keeping it in the instructor training lesson

### Which part of the content does your suggestion apply to?

https://carpentries.github.io/instructor-training/instructor/14-checkout.html

Compare instructor/14-checkout.html and instructor/checkout.html, then check the Handbook onboarding link to understand which page is intended as the canonical entry point. Resolve the duplicated checkout content and ensure the three checkout-step detail links remain discoverable without confusing duplicate pages.

Written by the indexing model from the issue text.