Document the envoy & proxy rebasing & building workflow

We should describe our rebase & build workflow for envoy & proxy in a well-written document so that anyone in the team can follow it as a guide and be able to produce a build for a new version of Maistra.

This work could be split into two main processes: community and product. For the community, this can be in the form of Markdown files within the "maistra" directory in both envoy and proxy projects in GitHub. For the product side, this could be a google document in our team drive.