Extend subject selection page

[lcjohnso]

The current subject selection page (https://help.zooniverse.org/next-steps/subject-selection/) includes a description of subject set and user chances/weighting. We can add descriptions of the following features here as well:

• Sequential Subject Selection: present subjects in defined order, via priority/#priority subject metadata key and prioritized workflow property (admin only); see https://stackoverflowteams.com/c/zooniverse/questions/291
• Subject Set Selection: volunteers select what subject set they want to work on via grouped workflow property (admin only); see https://stackoverflowteams.com/c/zooniverse/questions/383
• Indexed Subject Set Selection: volunteers select what subject they want to work on using the SubjectPicker; see https://stackoverflowteams.com/c/zooniverse/questions/370