This captures the API versioning guidelines implied by
https://docs.google.com/document/d/1xeVvJ6KjFBkNjVspPbY_PwEDHC7XPi0J5p1SqUXcCl8/edit#heading=h.xgk8xel154p


and splits apart the Envoy API and internal implementation breaking change policies.

Some of the policy decisions (e.g. not allowing vNalpha to be hand edited, how we do manual breaking
changes, etc.) have been added to these guidelines based on recent experience with protoxform and
mechanical major version upgrade work, they are not part of the original stable API versioning work.

Risk level: Low
Testing: Formatting and docs build.

Fixes #8371

Signed-off-by: Harvey Tuch <htuch@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 3801c6846a5c7bf4d230f1dad58985f2bb8bcdeb