Migrate "Lightweight Design" document for RO mobile application to GitLab

The document "Lightweight Design: A 'Read-Only' Tahoe-LAFS/PrivateStorage mobile application" currently lives on cryptpad at the following (view) address:

https://cryptpad.fr/pad/#/2/pad/view/4I1xA-GyP1M+c+Ss07z3rZCvjk8ovzebqZAqY4sinAA/

We should migrate this document to PrivateStorage's GitLab instance for all of the other reasons that one might want to use GitLab (e.g., so that we have an additional copy/backup of the work, so that we can track changes across time, so that we can submit pull-requests/merge-requests against it, etc.)