Add "Overall Plan.rst" document from the other repo.
Fixes #8 (closed).
Would be good to, at the same time as merging this, also remove the document from the other repo. I tried but failed to open a MR for that. I propose to replace the other doc with a note like this:
This now resides at the `privatestoragemobile repository /privatestorage/privatestoragemobile/-/blob/main/Overall%20Plan.rst`_.
Edited by Florian Sesser