Add initial architecture documentation (#1782)
Add initial architecture documentation (#1782)

This commit adds roughly the first 40% of the gopass
architecture documentation. Once completed this is supposed
to make it easier for new contributors to ramp up.

RELEASE_NOTES=n/a

Signed-off-by: Dominik Schulz <6770bd8ec89754c4b214547a9e0b59641dfbb9e3@gauner.org>
gopasspw/gopass,justwatchcom/gopass,franciscocpg/gopass,gopasspw/gopass