Categorization and annotation pages should include proper help text

[alyssadai]

The text should include:

• What the user is functionally doing on this page (mapping X to Y in the Neurobagel vocabulary, etc.)
• Any constraints on what needs to be filled out or mapped before the user can move on to the next step
• What they should click when they are done with that step (i.e., the "Next" button? or the next variable?)