Integrate a guide to explain the new Nutri-Score

[g123k]

Hi everyone!

Now that the new formula of the Nutri-Score is deployed in some countries, we need to deploy ASAP a guide to explain what's new. Here's the plan:

I have already designed and developed the screen (code here but not yet finalized). Here is a demo:
https://github.com/openfoodfacts/smooth-app/assets/246838/871dd454-4ebe-48b5-a30e-37d80c226cf8

For now, the content is already available in French and as we need to deploy it ASAP, we can't rely on what we do for other parts of the app. Basically, it will force us to wait for the translations OR release a few days later a new version of the app.

Instead, Crowdin has an API to fetch the translations.
So, I will create a separate ARB file that will be translated and fetched with the Crowdin API (there's a Flutter package).

Once this is done, here's the plan:

1. Finalized the code
2. Create a draft PR to validate the code
3. Provide access to this page via a deep link (TODO @teolemon if you have an idea of the URL)
4. Inject some popups / links in the app to give access to this screen (TODO @teolemon)
5. Create and publish a new release ASAP

[teolemon]

• We currently have /nutriscore which explains the methodology (which this page doesn't)
• It could be named new-nutriscore or nutriscore-v2 (cross-language), or the two could be merged.
• We need to translate all that to English
• For popups, I would imagine a bottom sheet promo on the scan page (or on the product page, whichever comes first), one time (with something reusable, because novelties that impact both product page and scan screen are bound to happen in the future

[g123k]

For a first step, we will show a popup

• If we open a product page
• AND if there is some history (= this is not the first visit)

We need to find a temporary way to have a static button somewhere in the app to access the guide