Move development docs from handbook to doc/dev

Created by: mrnugget

This is the mirror PR to https://github.com/sourcegraph/about/pull/2569.

So I quote the original PR description:

This is the result of a poll in Slack that I started in response to this entry in the "Developer experience issues we currently face" doc.

So far it seems that keeping practical dev documentation in this repository has more benefits than keeping them in the handbook.

There are still some higher-level docs here (our production environments, continuous delivery explanation, etc.) that I wasn't sure about.