Restructure the devguide for Jython

[jeff5]

Not recommended for merging until free of CPython specifics.

We'd like a Jython devguide based on CPython's, but the problem is that the CPython one is:

• shot through with CPython specifics we must replace, and
• likely to keep improving (which we'd like to merge).

Here I've tried to guess which files only need minor change. Two strategies have been applied, depending on how CPython specific was the original file:

• Where content applies to Jython, with simple edits, we keep the original file. The expectation is that subsequent merges will be automatic, or at leasst manageable manually.
• Where extensive re-writing is needed we start a new "companion" file (e.g. buildbots_jy.rst) and we keep the original as a reference, and also as a target for merge operations we don't want to modify the Jython text.

The aim is to allow changes to be merged as the CPython devguide develops.

index.rst is an exception as it is the conventional start page and cannot be renamed even though change is extensive.

[jeff5]

Well, it's not free of CPython specific text by any means, but I believe I've rescued all the text from @fwierzbicki's work and structured the guide suitably for our needs. Also, I want to see if I'm right that we are able to merge changes made to the CPython devguide, and what the process is ... which should be documented in the guide, of course. :)

I'll call PR done for now and merge soon.