[GNC] Project and Design Documentation

[flywire]

I've seen https://wiki.gnucash.org/wiki/Project_and_Design_Documentation
raised a bit in the gnucash-user mail list recently and I recall Geert's
comments about updating the csv importer and not documenting it. That wiki
hasn't effectively been updated for a decade with the meaningful part
written in html. I updated it to markup so it is not such an arduous task
to make a few comments.

It would be handy if someone with an oversight of development over the
last decade would update it to be more meaningful.

Regards