Improved spreadsheet templates for imports from manufacturers on the producers platform

[stephanegigandet]

@manon-corneille and the producers team are proposing improved templates that manufacturers can fill with their product data, that can then be easily imported on the producers platform.

Slack discussion: https://openfoodfacts.slack.com/archives/CK3AFTUFK/p1706180384187269

Template proposal: https://docs.google.com/spreadsheets/d/148PWIsSfDRsj8gLJ7cHbQtdapIEscUwX/edit#gid=1221199414

The current version of the template is generated with this link:
https://world.pro.openfoodfacts.org/cgi/generate_sample_import_file.pl
and it can be generated in different languages (e.g. French):
https://fr.pro.openfoodfacts.org/cgi/generate_sample_import_file.pl

We want to improve the generation of that template so that it looks like the proposed template.

The improvements can be done independently, so I'm opening separate issues:

Improvements to templates

Preview Give feedback

1. Improve the style of the header and cells of the import spreadsheet template on the producers platform #9873
   good first issue 🏭 Producers Platform 🏭 Producers Platform - data imports +3
2. Add a description line to the import spreadsheet template on the producers platform #9879
   good first issue 🏭 Producers Platform +2
3. Add an example line to the import spreadsheet template on the producers platform #9874
   good first issue 🏭 Producers Platform +2
4. Generate an import spreadsheet template with mandatory fields only on the producers platform #9876
   good first issue 🏭 Producers Platform +2
5. Add the list of all categories in the taxonomy to the import spreadsheet template on the producers platform #9877
   good first issue 🏭 Producers Platform +2

To test your changes in a local dev environment once you have followed the quick start guide https://github.com/openfoodfacts/openfoodfacts-server/blob/main/docs/dev/how-to-quick-start-guide.md , you can click on http://world.openfoodfacts.localhost/cgi/generate_sample_import_file.pl to generate the spreadsheet template.

[teolemon]

I reckon this is fixed @stephanegigandet @manon-corneille ?