docs: update info for learner home

[nsprenkle]

Add updated screenshots & descriptors for Learner Home initiative (new Learner Dashboard).

Date Needed (optional)

HOLD until widespread release of Learner Home

Reviewers

Possible roles follow. The PR submitter checks the boxes after each reviewer finishes and gives :+1:.

• [x] Subject matter expert: @mattcarter
• [ ] Doc team review (sanity check, copy edit, or dev edit?): @edx/doc
• [ ] Product review: Ryan O'Connell

FYI: Tag anyone else who might be interested in this PR here.

Testing

• [ ] Ran ./run_tests.sh without warnings or errors

HTML Version (optional)

• [ ] Build an RTD draft for your branch and add a link here

Post-review

• [ ] Add a comment with the description of this change or link this PR to the next release notes task.
• [ ] Squash commits