docs: event tag migration guide

[Roiocam]

References #731

[octonato]

I left some rewording suggestions.

I know realise that the way we are creating the migration script wouldn't allow for proper rolling upgrades.

To have it properly done, a user must first run a script that only creates the new columns.

Then they need to redeploy their application in legacy-mode. The application will start to write on all three columns. But the join will be done using the event_id.

After that, they should run the second part of the script that will migrate the rows and adapt the constraints. Once this is done, they can redeploy their application by disabling the legacy-mode.

[Roiocam]

@octonato I have update the guide, Explicitly point the mutiple-phase rollout/deploy, and the two configuration style.

The script has update this too: 8587ba0

Can you take a look? Thanks.

[octonato]

Thanks @Roiocam

Looking good. Left a few minor change requests.
I think we can merge it after that.

[Roiocam]

Thanks @Roiocam

Looking good. Left a few minor change requests. I think we can merge it after that.

Thanks for the feedback.