Define BB upgrade path/guidance

Currently Big Bang has no documentation around how to complete an upgrade. We should provide a minimal upgrade path document that walks through:

• How to upgrade a single package (git.tag / oci.tag override): Capture that packages can be updated one at a time to stay current, release notes review for individual packages + upstream review for breaking changes.
• How to upgrade BB umbrella: Review of release notes for any upgrade notices, known issues, etc. What does an actual code change look like to upgrade (focus on customer template usage).
• How to know your upgrade "worked": What to look for in cluster to validate that Big Bang has upgraded and is healthy
• What we support with respect to version jumps: Can I upgrade from 1.2.0 -> 1.55.0? We should especially call out that we generally expect people to upgrade from n-1, but provided people review release notes in between jumping several versions is doable. We should also include a mention that we recommend testing upgrades in a staging environment that mirrors prod to catch any errors early.