EraTransitionGovernance.md: first draft

[nfrisby]

This document is intended to help teams implement testnets without relying on TestXxxHardForkAtEpoch, ie as a step towards the latest plan for Issue 416.

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Rendered: https://github.com/IntersectMBO/ouroboros-consensus/blob/nfrisby/how-to-era-transition/docs/website/contents/for-developers/EraTransitionGovernance.md

[nfrisby]

@sgillespie @johnalotoski @mkoura

I've drafted a document here in this PR that provides the top-level advice for how to increment the major protocol version with each era, and when doing so will cause a transition to the next era.

Would you review it? In particular, I'd like to know which aspects of the process (of introducing the right transactions at the right time instead of TestXxxHardForkAtEpoch) you're still left wondering about after reading this short document. Thanks!

Edit: also some of you already have concrete experience getting these transactions to work, so please suggest any knowledge you already have that should be added to this document. EG something about making it as simple/easy as possible to arrange for the necessary transactions' fees.

[nfrisby]

I added a section that explains the TestXxxHardForkAtEpoch: 0 feature.

[nfrisby]

And another section that links to and summaries the Byron, Shelley...Babbage, and/or Conway governance demonstrated by a few key test suites.

This is now the first full draft of the document.