Broken links in docs.

[thorwhalen]

The links under the "Layers" RPython subsection are all broken in some way.

The first, :ref:RPython <rpython:language>, points to https://github.com/mozillazg/pypy/blob/master/pypy/doc/architecture.rst#id15, which is just the same page.

The second, Getting Started with RPython, points to, which leads to a "Sorry This page does not exist yet".

The third, RPython By Example, points to https://mesapy.org/rpython-by-example/, which 404s