Consolidate developer documentation for securedrop-builder

[sssoleileraaa]

Description

As part of freedomofpress/securedrop-builder#304, securedrop-debian-packaging has been renamed to securedrop-builder so this repo name is now outdated. Scanning through the guide, I see that it provides generic debian packaging help for python projects, so I also don't want to just rename it to securedrop-builder-guide, which might mislead folks. Also, our dev docs for securedrop-builder will soon reside in https://github.com/freedomofpress/securedrop-dev-docs.

@legoktm - what do you think we should migrate over to our dev docs before we archive this repo? Or do you think we should keep it around and continue to link to it (see https://github.com/freedomofpress/securedrop-builder#intro-to-packaging).

[maeve-fpf]

Crosslinking freedomofpress/securedrop-builder#399 - looks like this is currently hosted on RTD. Will it be folded into the developer docs that are self-hosted?

[legoktm]

We should definitely fold it into securedrop-dev-docs. Unfortunately most of the content seems outdated, I can review it in a bit to see what is worth copying over.