Refactor UI text for To-Do List empty state

What does this MR do and why?

Improves wording and fixes links on the empty state for the To-Do List.

Closes: Improve empty state for Todos (#429827 - closed)

Screenshots or screen recordings

Before	After

Links:

Before	After
open issues: links to https://gitlab.com/dashboard/issues?sort=created_date&state=opened, which is always empty for me	assigned issues: links to https://gitlab.com/dashboard/issues?state=opened&assignee_username[]=current-user
a merge request: links to https://gitlab.com/dashboard/merge_requests?assignee_username=current-user	merge requests: no change in URL
	What actions create to-do items?: links to https://docs.gitlab.com/ee/user/todos.html#actions-that-create-to-do-items

How to set up and validate locally

In GDK, go to http://<gdk URL>/dashboard/todos and make sure it's empty.

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.