Dev docs: Debian 9 -> Debian "stable"
Dev docs: Debian 9 -> Debian "stable"

We'll continue to track the stable series of Debian (currently 10,
Buster) for developer docs, so use the "stable" label.

conorsch/securedrop,conorsch/securedrop,conorsch/securedrop,conorsch/securedrop,conorsch/securedrop