doc/developers vs doc/en/developer-guide/

[Ian Clatworthy]

Robert Collins wrote:
> Can we nuke the latter?

I'd rather not. It's there so that all of the core docs have a
consistent location in the generated doc tree, namely

  language-code/doc-name/.

I believe quite a few developers/users are looking to translate the core
docs to other languages and I'd like to see the Developer Guide included
in that.

Ian C.