Simplify developer flow

• Automate the release of the Gem #45
• Generate automatic documentation from code

Add new and missing features

• New APIs: https://github.com/openfoodfacts/api-documentation/issues?q=is%3Aissue+is%3Aopen+label%3A%22new+api+to+document%22
• Existing APIs: https://github.com/openfoodfacts/api-documentation/