Add GA networking spec (#615)

docs(sandbox-policy): add v2 networking GA spec

Convert the reviewed "MXC Network Configuration, GA" Word doc into
Markdown and check it in at docs/sandbox-policy/v2/networking.md
(connectivity models, proposed schema, design decisions D1-D8,
per-backend GA scope, gaps and limitations).

The Appendix: Industry Precedent and References sections and all
inline references to external agent runtimes are omitted, since they
are not needed in the repo.

Author: bbonaby