Tutorial refactor: move/update the "Contact Participants" info

[becky-gilbert]

This addresses part of #436 (but doesn't close the issue).

This PR takes the main instructions for contacting/messaging participants out of the tutorial and moves them to a new "Contacting participants" section in the "Day-to-day study operations" page. The tutorial still contains some exercises related to messaging participants but it links back to the new "Contacting participants" section. This should make this information easier to find as it was previously just "buried" in the tutorial.

This PR also does the following:

• Updates out-of-date screenshots related to participant messaging and adds a few new ones.
• Adds links to the new "Contacting participants" section to make it easier to find.
• Updates and clarifies text on the "Day-to-day study operation" page related to study status, discoverability, and announcement emails. This includes explaining where exactly to go to change a study's "discoverability" in that section (this was part of the feedback in UPDATE, STRUCTURE - Sienna tutorial feedback #436).