dev/sg: migrate sg docs to doc/dev
Created by: bobheadxi
This PR moves sg
documentation to doc/dev
so that it shows up in the docsite and is more easy to discover via search.
More specifically, gives sg
a home under dev/background-information/sg
aka https://docs.sourcegraph.com/dev/background-information/sg (why is this URL so long? maybe it could be just background
but oh well)