Upgrade workflow

Node operator doc work by Mina Foundation for the major upgrade

TLDR;

1. Fork the https://github.com/o1-labs/docs2 repository.
2. Create PRs against the major-upgrade (https://github.com/o1-labs/docs2/tree/major-upgrade) branch, not main.
3. Be sure to keep the major-upgrade branch current.
4. All PRs for the major upgrade get merged into the major-upgrade branch.
5. When it's time, a single PR that includes all of the updates gets merged into main.

O(1) Labs looking into possible upstream automation to keep the major-upgrade branch current with main.