Cross-link and fix to-do items API page

What does this MR do?

As part of !53377 (merged) I needed to cross-link to information about to-do items. When I searched, the first result was the To Dos API https://docs.gitlab.com/ee/api/todos.html page, which had no crosslink back to the main page about to-do items. Let's fix that.

While this page is technically devopsplan groupproject management —the impetus was from Create, and so I'm sending it over to Kai.

Related issues

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