Re-examine re-use of the CPython dev guide

[jeff5]

I'm convinced we benefit by re-using the CPython guide, but the idea was to track change by pulling in CPython changes as we go along. In practice, this doesn't happen, and when a backlog has built up, the merge looks scary.

Chapters are files. There are three cases to consider:

1. A file is the same in Jython as in CPython (yay!).
2. A file has a small number of differences in Jython's copy from CPython's. One might be able to have a file of the same name and merge the changes.
3. A file differs a lot. Jython needs its own version entirely. (Include here chapters we drop and chapters we add.)

In practice, 2. is where the problem lies, as it's always more important to make a specific change than keep up.

[jeff5]

A problem observed with category 3. is as follows. Where there is a "completely different" version of an existing CPython chapter (e.g. runtests_jy.rst replaces runtests.rst) there remains a lot of cross-referencing under the old name (or labels defined in the original). This leads readers into trouble.

It is better not to have both versions around, and to re-use labels (where they mean the same), or invalidate them (which alerts us to dangling references through an error). Corresponding chapters then have the same file name.