Add migration guide

[qmuntal]

We should have a migration guide to help 1P customers migrate from upstream Go to Microsoft Go.

This guide should at least contain the following information:

• How to acquire Microsoft Go
• Microsoft Go divergences with upstream Go
• Links to any existing useful documentation

[gdams]

I'll take a first pass

[dagood]

An angle to consider here is various forms of migration from "where they are", e.g. minimal, distroless, or scratch Azure Linux 3 containers. Making sure we say that OpenSSL must be installed and SymCrypt/SCOSSL packages, for example. (And perhaps troubleshooting notes: what panics occur for various situations.)