PR template for lesson content with checkboxes

We're starting to standardize around what we expect within lesson content. Let's codify those items as checkboxes within a PR template, so that they remain front and center.

Check the repository's existing pull-request template locations and compare them with the linked academy-turbo V2 repository, since this repository is marked deprecated. Done means the appropriate active project has a lesson-content PR template containing the agreed checklist items as checkboxes.

Written by the indexing model from the issue text.