Adds the epic for infinite scroll

What does this MR do and why?

This adds the migration guide for infinite scroll. We already had it, but it was written in a ruleset rather than an actual epic.

Review/run this locally

1. Copy the pages in mr job URL (must have succeeded)
2. Run bin/review-mr.sh <job url>
3. If there are rules changes, check the compare findings job log reports the expected changes

Closes #111 (closed)