Add a note to the deploy instructions and README about the `production` branch

[awwaiid]

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Context for documentation change

We decided to add a production branch to track "what is currently running in production". We already have a current-production tag, but it's nice to have a permanent name (which is what branches are for!). So document that to cut a deploy we:

• Merge main -> production
• Tag the release version number (the current date)
• Push with tags!

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