GeezerVison

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. Where extensive re-writing is needed we start
a new "companion" file (e.g. grammar_jy.rst) and keep the original for
reference.

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.

These are words specific to Jython and Mercurial, merged by hand. We
restore some sections the CPython version has lost or never had. Much is
still in place that is CPython-specific.

Check and fix hyperlinks so the reader is not carried into material
specific to CPython while navigating. Also fix up content at the
destination sections, where reasonably easy to do.

Check and fix hyperlinks (all the :ref: and :doc: formulae) so the
reader is not carried into material specific to CPython while
navigating.

Speak distinctly about Jython in a coherent section. Keep (and update)
the CPython story exactly as that project tells it.

Just too much divergence to remain common.