Document backup/restore format

[kdmccormick]

@ormsbee , we still need to do this, correct? I can put it in the Open edX Core v1 epic if so

This task includes:

• [verawood] Legacy Libraries DEPR: Document V1->V2 export format changes frontend-app-authoring#2877

[kdmccormick]

Moving this down in priority since it could be done after the Verawood cutoff (but before the release)